Patent application title: CALIBRATION DEVICE FOR USE IN AN OPTICAL PART MEASURING SYSTEM

Abstract:

A calibration device for use in an optical, part measuring system is
provided. The device has a central axis and a plurality of regions which
are rotationally symmetric about the axis. The device includes a series
of step-shaped portions defining a multi-step region having a plurality
of step edges. A profile of the multi-step region contains information
for calibrating the system. The device further includes a plurality of
cylindrically-shaped portions spaced apart along the axis and defining
constant diameter regions containing information for calibrating the
system. The device still further includes a frustum-shaped portion
defining a pair of spaced, slope edge regions and a sloped region having
boundaries marked by the pair of slope edge regions. The frustum-shaped
portion has first and second diameters at its boundaries which define a
range of diameters of parts capable of being measured in the system. A
profile of the slope edge regions and the sloped region contains
information for calibrating the system.

Claims:

1-31. (canceled)

32. A calibration device for use in an optical, part measuring system, the
device having a height along a central axis and at least one region which
is rotationally symmetric about the axis, the device comprising:a series
of step-shaped portions defining a multi-step region having a plurality
of step edges wherein at least one of the step-shaped portions begins at
the same height along the axis that a respective adjacent step-shaped
portion ends along the axis at one of the step edges and wherein a
profile of the multi-step region contains information for calibrating the
system.

33. The device as claimed in claim 32, wherein the device is NIST
traceable.

34. The device as claimed in claim 32 further comprising at least one
cylindrically shaped portion defining a constant diameter region
containing information for calibrating the system.

35. The device as claimed in claim 32 further comprising a plurality of
cylindrically-shaped portions spaced apart along the axis and defining
constant diameter regions containing information for calibrating the
system.

36. The device as claimed in claim 32 further comprising a support portion
for supporting the device within the system.

37. The device as claimed in claim 34, wherein the at least one
cylindrically-shaped portion defines a begin edge region at a distal end
of the device.

38. The device as claimed in claim 32, wherein the step edges are equally
spaced along the axis.

39. The device as claimed in claim 32, wherein each of the step-shaped
portions is ring-shaped.

40. The device as claimed in claim 32 further comprising an end portion
for supporting part support apparatus at an end of the device.

41. The device as claimed in claim 32 further comprising an end portion
having a central hole defining a center of the device for receiving part
support apparatus therein at an end of the device, the central hole also
defining a center of the part support apparatus.

42. A certified precision calibration cone for use in an optical, part
measuring system, the cone having a height along a central axis and at
lease one region which is rotationally symmetric about the axis, the cone
comprising:a series of step-shaped portions defining a multi-step region
having a plurality of step edges wherein at least one of the step-shaped
portions begins at the same height along the axis that a respective
adjacent step-shaped portion ends along the axis at one of the step edges
and wherein a profile of the multi-step region contains information for
calibrating the system.

43. The cone as claimed in claim 42, wherein the cone is NIST traceable.

44. The cone as claimed in claim 42 further comprising at least one
cylindrically-shaped portion defining a constant diameter region
containing information to calibrate the system.

45. The cone as claimed in claim 42 further comprising a plurality of
cylindrically-shaped portions spaced apart along the axis and defining
constant diameter regions containing information for calibrating the
system.

46. The cone as claimed in claim 42 further comprising a support portion
for supporting the cone within the system.

47. The cone as claimed in claim 44, wherein the at least one
cylindrically-shaped portion defines a begin edge region at a distal end
of the cone.

48. The device as claimed in claim 42, wherein the step edges are equally
spaced along the axis.

49. The device as claimed in claim 42, wherein each of the step-shaped
portions is ring-shaped.

50. The cone as claimed in claim 42 further comprising an end portion for
supporting part support apparatus at an end of the cone.

51. The cone as claimed in claim 42 further comprising an end portion
having a central hole defining a center of the cone for receiving part
support apparatus therein at an end of the cone, the central hole also
defining a center of the part support apparatus.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is related to the following commonly-owned U.S.
patent applications which were filed on Oct. 23, 2007: [0002]1) Method
and System for Optically Inspecting Parts U.S. Ser. No. 11/977,117);
[0003]2) Method for Estimating Thread Parameters of a Part U.S. Ser. No.
11/977,097) now U.S. Pat. No. 7,633,046; [0004]3) Optical Modules and
Method of Precisely Assembling Same U.S. Ser. No. 11/977,102) now U.S.
Pat. No. 7,633,634; [0005]4) Method and Inspection Head Apparatus for
Optically Measuring Geometric Dimensions of a Part U.S. Ser. No.
11/977,010); [0006]5) Apparatus for Quickly Retaining and Releasing Parts
to be Optically Measured U.S. Ser. No. 11/977,091); and [0007]6) Method
and System for Generating Calibration Data For Use In Calibrating A Part
Inspection System U.S. Ser. No. 11/975,977). [0008]7) Calibration Device
For Use In An Optical Part Measuring System (U.S. Ser. No. 11/977,114).

BACKGROUND OF THE INVENTION

[0009]1. Field of the Invention

[0010]This invention relates to calibration devices for use in optical,
part measuring systems.

[0011]2. Background Art

[0012]Traditional manual, gauging devices and techniques have been
replaced to some extent by automatic inspection methods and systems.
However, such automatic inspection methods and systems still have a
number of shortcomings associated with them.

[0013]WO 2005/022076 discloses a plurality of light line generators (72)
which generate associated beams of light (26) that intersect a part (14)
to be inspected. Each beam of light (26) illuminates at least one side of
the part (14) with a line of light occluded by the part (14), and at
least three light responsive sensors (104) provide for generating a
signal (24) responsive to an occlusion of a corresponding line of light
on a corresponding side of at least one side of the part (14). Each of
the light responsive sensors is responsive to an occlusion at a different
azimuthal location. A processor (28) analyzes the signals (24) in
relation to a measure of relative location of the part (14) from a motion
(18) or position sensor. The part (14) may be released from a clamp (52)
to drop through the beams of light (26), or the beams of light (26) may
be moved relative to the part (14).

[0014]U.S. Pat. No. 6,313,948 discloses an optical beam shaper for
production of a uniform sheet of light for use in a parts inspection
system having a light source including a coherent light generator, a
diffractive beam shaper, and lens elements.

[0015]U.S. Pat. No. 6,285,031 discloses an inspection system for
evaluating rotationally asymmetric workpieces for conformance to
configuration criteria. The system has a track for causing the workpieces
to translate through a test section. The test section includes a
plurality of electromagnetic energy sources. The plurality of
electromagnetic energy sources are oriented with respect to the track
such that the workpieces occlude the plurality of electromagnetic energy
sources upon passing through the test section. The test section further
has electromagnetic energy detectors for receiving the electromagnetic
energy to provide output signals related to the intensity of the occluded
electromagnetic energy incident on the electromagnetic energy detectors,
and a signal processor for receiving and processing the output signals.

[0016]U.S. Pat. No. 6,252,661 discloses an inspection system for
evaluating workpieces for conformance to configuration criteria. The
system includes a track for causing workpieces to translate through a
test section. The test section includes a light source for producing a
uniform sheet of light. The light source is oriented with respect to the
track such that the workpieces occlude the uniform sheet of light upon
passing through the test section. The test section further has a video
system for receiving the occluded uniform sheet of light, providing
output signals related to the intensity of the occluded uniform sheet of
light incident on the video system, and a signal processor for receiving
and processing the output signals.

[0017]U.S. Pat. No. 6,959,108 discloses an inspection system wherein
workpieces to be inspected are consecutively and automatically launched
to pass unsupported through the field of view of a plurality of cameras.
As a workpiece passes through the field of view of the cameras, a sensor
is activated which communicates with a computer system to activate the
cameras to capture an unobstructed image, or image data, of the
workpiece. The image data is then analyzed by a computer program to
verify whether the image data indicates that the workpiece does not meet
established criteria and therefore is considered defective. If the image
does not meet the established criteria, the workpiece is rejected and
segregated from workpieces which have not been identified as defective.

[0018]U.S. Pat. No. 5,608,530 discloses a laser for producing a beam of
radiation which is then refined in cross-sectional dimension by use of
plano-cylindrical lenses. The refined beam of radiation falls incident on
a part to be measured. The unobstructed portion of the beam is then
bifurcated by a pair of reflective surfaces which produce non-parallel
radiating beams. Each resulting beam comprises the unobstructed portion
of radiation which has passed radially opposed halves of the part. The
magnitude of radiation present in each non-parallel radiating beam is
then measured.

[0019]U.S. Pat. No. 4,831,251 discloses an optical device for
discriminating threaded workpiece by the handedness by their screw thread
profiles. The device present a pair of light beams which pass generally
tangent to the workpiece at angularly displaced positions. The light
beams are inclined to follow the helix direction of a given handedness of
a workpiece. Upon axial advancement of a workpiece through the device, a
chopped output from the photodetectors indicates that the handedness of
the threads matches the inclination of the light beams. The oppositely
threaded workpiece, however, provides a generally constant DC output.
With appropriate signal processing electronics, an automatic system for
discriminating workpieces by thread handedness is provided.

[0020]U.S. Pat. No. 5,383,021 discloses a non-contact inspection system
capable of evaluating spatial form parameters of a workpiece to provide
inspection of parts in production. The system causes parts to be
sequentially loaded onto an inclined track where they pass through a test
section. The test section includes a length detection array for measuring
the length of the workpiece, which includes a source generating a sheet
of light oriented in the longitudinal direction of the workpiece. The
profile of the parts are evaluated by one or more light sources also
creating a sheet of light oriented transversed to the longitudinal axis
of the parts. Single channel photodetectors are provided for each of the
sources which provides an analog output of the extent to which each sheet
of light is occluded by the part. These outputs are analyzed through
appropriate signal processing hardware and software to generate length
and profile data related to the workpiece geometry.

[0021]U.S. Pat. No. 5,568,263 discloses a non-contact inspection system
capable of evaluating spatial form parameters of a workpiece to provide
inspection of parts in production. The system causes parts to be
sequentially loaded onto an incline track where they pass through a test
section. The test section includes a length detection array for measuring
the length of the workpiece, which includes a source generating a sheet
of light oriented in the longitudinal direction of the workpiece. The
profile of the parts are evaluated by one or more light sources also
creating a sheet of light oriented transverse to the longitudinal axis of
the parts. First and second pairs of single channel photodetectors are
provided for each of the light sources which provides a pair of analog
outputs of the extent to which each sheet of light is occluded by the
part, as well as an ability to eliminate noise or scintillation caused by
a point source of light, for example with a laser light source. These
outputs are analyzed through appropriate signal processing hardware and
software to generate length and profile data related to the workpiece
geometry.

[0022]U.S. Pat. No. 4,852,983 discloses an optical system which simulates
the optical effect of traveling over a large distance on light traveling
between reference surfaces.

[0023]U.S. Patent Application Publication No. 2005/0174567 discloses a
system to determine the presence of cracks in parts. The presence of
cracks is determined through the use of an imaging device and
illumination source. The part is moved along a track where it is sensed
by a position sensor to initiate the inspection. The illumination source
projects a sheet of light onto the part to be inspected. The line formed
by the intersection of the sheet of light and the part is focused onto
the imaging device. The imaging device creates a digital image which is
analyzed to determine if cracks are present on the part.

[0024]U.S. Patent Application Publication No. 2006/0236792 discloses an
inspection station for a workpiece including a conveyor, a mechanism for
rotating the workpiece, and a probe. The conveyor includes a fixture for
locating the workpiece and the conveyor is configured to translate the
workpiece in a linear manner A mechanism, such as a belt, engages the
workpiece thereby rotating the workpiece within the fixture. The probe is
configured to indicate if the workpiece conforms to quality criteria. To
facilitate inspection while the conveyor translates the workpiece, the
probe is attached to a stage where the stage is configured to move the
probe synchronously with the workpiece over an inspection region.

[0027]An object of the present invention is to provide an improved
calibration device for use in an optical part measuring system.

[0028]In carrying out the above object and other objects of the present
invention, a calibration device for use in an optical, part measuring
system is provided. The device has a central axis and a plurality of
regions which are rotationally symmetric about the axis. The device
includes a series of step-shaped portions defining a multi-step region
having a plurality of step edges. A profile of the multi-step region
contains information for calibrating the system. The device further
includes a plurality of cylindrically-shaped portions spaced apart along
the axis and which define constant diameter regions which contain
information for calibrating the system. The device still further includes
a frustum-shaped portion defining a pair of spaced, slope edge regions
and a sloped region having boundaries marked by the pair of slope edge
regions. The frustum-shaped portion has first and second diameters at its
boundaries which define a range of diameters of parts capable of being
measured in the system. A profile of the slope edge regions and the
sloped region contain information for calibrating the system.

[0029]The device may further include a second frustum-shaped portion which
defines a second pair of spaced, slope edge regions and a second sloped
region which may have boundaries marked by the second pair of slope edge
regions.

[0030]The second frustum-shaped portion may have first and second
diameters at its boundaries which may define a second range of diameters
of parts capable of being measured in the system. A profile of the second
pair of slope edge regions and the second sloped region may contain
information for calibrating the system.

[0031]Further in carrying out the above object and other objects of the
present invention, a calibration device for use in an optical, part
measuring system is provided. The device has a central axis and a
plurality of regions which are rotationally symmetric about the axis. The
device includes a series of step-shaped portions defining a multi-step
region that has a plurality of step edges. A profile of the multi-step
region contains information for calibrating the system. The device
further includes a plurality of cylindrically-shaped portions spaced
apart along the axis and which define constant diameter regions
containing information for calibrating the system. The device still
further includes a first frustum-shaped portion which defines a first
pair of spaced, slope edge regions and a first sloped region which has
boundaries marked by the first pair of slope edge regions. The first
frustum-shaped portion has first and second diameters at its boundaries
which define a first range of diameters of parts capable of being
measured in the system. A profile of the first pair of slope edge regions
and the first sloped region contains information for calibrating the
system. The device includes a second frustum-shaped portion which defines
a second pair of spaced, slope edge regions and a second sloped region
which has boundaries marked by the second pair of slope edge regions. The
second frustum-shaped portion has first and second diameters at its
boundaries which define a second range of diameters of parts capable of
being measured in the system. A profile of the second pair of slope edge
regions and the second sloped region contain information for calibrating
the system.

[0032]The device may be NIST traceable.

[0033]One of the cylindrically-shaped portions may mark a boundary between
the sloped regions.

[0034]The device may include a cylindrically-shaped portion which defines
a maximum, constant diameter region which, in turn, may mark a boundary
between the second sloped region and the multi-step region.

[0035]The device may further include a support bracket portion which
supports the device within the system.

[0036]The support bracket portion may suspend the device within the
system.

[0037]One of the cylindrically-shaped portions may define a begin step
edge region at a distal end of the device.

[0038]The step edges may be equally spaced along the axis.

[0039]Two of the constant diameter regions may have equal diameters.

[0040]The first sloped region may have a constant slope.

[0041]The second sloped region may have a constant slope.

[0042]Each of the sloped regions may have a constant slope and wherein the
slopes may be equal.

[0043]Each of the step-shaped portions may be ring-shaped.

[0044]Each of the cylindrically-shaped portions may be ring-shaped.

[0045]The device may include a ring-shaped portion for receiving part
support apparatus therein at a proximal end of the device.

[0046]The above object and other objects, features, and advantages of the
present invention are readily apparent from the following detailed
description of the best mode for carrying out the invention when taken in
connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a schematic perspective view of a part inspection system
including measurement hardware;

[0048]FIG. 2 is a schematic, side elevational view of a part holder base
(with a side plate of a container removed) and an upper tooling unit with
a part held between the units and a calibration fixture or device mounted
colinear with the part;

[0049]FIG. 3a is a perspective schematic view of the part holder base with
a drive bit supported thereon and the calibration device suspended
therefrom;

[0050]FIG. 3b is a top plan view of the base of FIG. 3a;

[0051]FIG. 3c is a sectional view taken along lines 3c-3c of FIG. 3b;

[0052]FIG. 4a is a side elevational view of the calibration device or cone
of FIG. 3a;

[0053]FIG. 4b is a top plan view of the cone of FIG. 4a;

[0054]FIG. 4c is a sectional view taken along lines 4c-4c of FIG. 4b;

[0055]FIG. 5 is a top plan view of an optical head with its top cover
plate removed in order to provide an interior view of the optical head;

[0056]FIG. 6 is a schematic perspective view of a safety enclosure which
may enclose the basic measurement hardware of FIG. 1;

[0057]FIG. 7 is a side elevational view, partially in cross-section, of
the upper tooling unit of FIG. 2;

[0058]FIG. 8 is a schematic block diagram which illustrates basic beam
line subsystem components wherein a laser generates a laser beam, a
mirror reflects the laser light beam, and a light plane generator module
generates a laser light plane which is directed or projected onto a part;

[0059]FIG. 9 is a schematic perspective view of a cylindrical part which
is intersected by a projected laser light plane;

[0060]FIG. 10 is a schematic view, partially broken away, of various
position measurement system components together with the optical head
which is mounted on a stage to move therewith just prior to scanning the
calibration device or cone;

[0061]FIG. 11 are graphs of various raw sensor signals generated when the
beginning edge of the calibration cone is scanned;

[0062]FIG. 12 is a schematic block diagram of various laser transmitters
and receivers contained within the optical head;

[0063]FIG. 13 is a top plan schematic view of a part illuminated by
multiple planes of laser light with various tangential shadow rays and
points;

[0064]FIG. 14 is a graph of sensor height data created with a threaded
part such as a standard thread plug go gage;

[0065]FIG. 15 is a symbolic sketch of the calibration cone's profile with
various usage regions;

[0067]FIG. 17 is a thread model superimposed on a graph which illustrates
rough thread peaks, troughs (or more correctly "crests" and "roots") and
crossings obtained by the intersection of a pitch diameter line with
flank lines;

[0068]FIG. 18 is a graph similar to the graph of FIG. 17 wherein selected
thread concepts are illustrated;

[0069]FIG. 19 is a single thread form which illustrates thread flank
lines, a thread flank line data extraction region and a wire position;

[0070]FIG. 20 is a schematic diagram which illustrates a 3-point distance
measurement between a reference line defined by a pair of virtual wire
centers on one side of a threaded part and a single wire center on the
opposite side of the part;

[0071]FIG. 21 is a screen shot of a graph of sensor height data with
virtual wires in roots and with intermediate data illustrated;

[0072]FIG. 22 is a screen shot which illustrates an enlarged thread pitch
with intermediate data;

[0073]FIG. 23 is a top plan schematic view of a cylindrical part with a
beam of light deflected at an angle θrefl=2θ by a
perfectly reflecting surface of the part;

[0074]FIG. 24 is a top plan schematic view of a part which scatters light
from a plane of light which scattered light is blocked by light plane
receiver aperture slits;

[0076]FIG. 26 is a sketch, partially broken away, similar to the sketch of
FIG. 15 which shows a schematic outline of the sensor signal produced by
the calibration cone and various support structures;

[0077]FIG. 27 is a sketch which illustrates a full open signal level
computed from data in the full open estimation region;

[0078]FIG. 28 is a graph which shows the data regions of FIG. 27;

[0079]FIG. 29 is a sketch which illustrates cone projection geometry;

[0080]FIG. 30 is a sketch of α1, α2, α3,
α4 which are the projections of {right arrow over (α)}
on the y'-axis for the θ=22.5, 67.5, 112.5, 157.5 degree laser
sensor systems; the positive y'-axis is the right sensor and the negative
y'-axis is the left sensor direction;

[0081]FIG. 31 is a schematic perspective view of an adjustment fixture for
assembling optical modules;

[0096]FIG. 46a-46d are top plan views of the assembly of FIG. 45 and
having different angular portions for use in the optical head of FIG. 5;
and

[0097]FIG. 47 is an exploded perspective view of a typical receiver module
including its receiver mount and its supported set of optical components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0098]The overall system described herein is often referred to as "Laser
Lab." Laser Lab is a trademark of the assignee of this application. It is
to be understood that numerous inventions are described herein, only some
of which are claimed herein. The other disclosed inventions are claimed
in the applications noted in the Cross Reference to Related Applications
part of this application. It is also to be understood that a number of
words and phrases are explained in a Glossary portion of this
application. The Glossary explains but does not unduly limit the words
and phrases contained therein.

Laser Lab--Physical Overview

[0099]The Laser Lab system (i.e., FIG. 1) includes several physical
subsystems or units.

[0100]A PC tower unit (i.e., FIG. 8) contains a computer and a number of
additional control electronics modules. It has a rear panel with
connectors for light curtain/safety electronics, motor control, linear
encoder, measurement signals, and optical head control. The PC tower unit
hosts the application program which presents a user interface to an
operator of the Laser Lab system.

[0101]Part holder and upper tooling units (FIGS. 1, 2 and 7) secure or
receive and retain a part in place for measurement. The upper tooling
unit includes a stainless steel rod with a spring loaded tip that can
move up and down to accommodate a wide variety of part sizes. The part
holder unit has a base to support the part or unit under test (UUT) and a
calibration cone or device (i.e., FIGS. 3a-4c). The calibration cone is
used to measure the relationship between a light sensor output and the
physical measurements represented by the outline dimensions of the cone.
The calibration cone or device is not strictly speaking a cone but rather
includes a number of frustums (i.e., portions of cones) and cylinders.

[0102]An optical head (i.e., FIGS. 1, 5 and 10) is a sealed system
containing a number of components including optical measurement
components (i.e.,

[0103]FIGS. 5 and 12). A set of (4) laser beam lines (one of which is
shown in FIG. 8) generate and measure 4 planes of light.

[0104]A slide/base unit (i.e., FIGS. 1 and 10) moves the optical head
vertically up and down to make part measurements. On every scan the
optical head's (8) sensors measure shadow images of both the calibration
cone and of the part (UUT) (i.e., FIGS. 8, 9 and 13). Each complete scan
thus contains both calibration data and measurement data, yielding a
system that is especially immune to time variations in measurement
conditions.

[0105]Referring again to the drawing figures, FIG. 1 is a schematic
perspective view of the Laser Lab system, generally indicated at 10,
including the basic measurement hardware of the system 10. Shown are the
optical head, generally indicated at 12, the part holder/upper tooling
units, generally indicated at 14 and 16, respectively, and the base/slide
unit, generally indicated at 18, which, as also shown in FIG. 10,
includes a motor 20 coupled to a lead screw 22 which, in turn, is coupled
to a saddle 24 slidably supported by a bearing. The saddle 24 is coupled
to the optical head 12 to move the optical head 12 linearly along a
vertical stage axis 28 (i.e., FIG. 9). Movement of the stage is sensed by
a linear encoder 30 which will be described in greater detail
hereinbelow.

[0106]FIG. 2 is a schematic, side elevational view of the part holder and
upper tooling units 14 and 16, respectively. The upper tooling unit 16
includes a rod 32 which is manually movable along a central axis of the
rod 32 in up and down directions by an operator of the system 10. The
upper tooling unit 16 also includes a spring-loaded part clamp 34 having
a tip 35 which retains a part 36 to be inspected at one of its end
surfaces 38. Also illustrated in FIG. 2 are a calibration cone or device,
generally indicated at 40, and a base or top plate 42 of the part
holder/base unit 14.

[0107]FIG. 3a is a perspective schematic view of the part holder base unit
14. FIGS. 3a-3c show the calibration cone 40 and a part holder assembly
including a support 44, a collar 46, a bit holder 47 into which a Torx
bit 48 (i.e., Tx40) is inserted for engagement with a recessed portion of
the end surface. A thread plug gage (instead of a recess bit for a bolt)
may be provided. The end user screws on a nut all the way to the end of
the plug gage. This ensures the threads of the nut are good. Then the
system 10 measures the rest of the outer diameter characteristics of the
nut.

[0108]A cap 49 covers the assembly. FIGS. 3a-3c also show various
mechanical pieces or parts including a mounting plate 50, a riser 52, a
bottom plate 54, the top plate 42, an adapter plate 58 for suspending the
cone 40 at the lower surface of the top plate 42 and a pair of posts 56
for supporting the top plate 42 at a spaced position above the bottom
plate 54.

[0109]FIG. 4a is a side elevational view of the calibration cone 40. As
described in detail below, the calibration cone 40 has a precisely
manufactured shape that is utilized in measuring the relationship between
raw digitized sensor signals and calibrated physical dimensions.
Typically, the cone 40 is sent to a certified laboratory which inspects
the cone 40. The laboratory then provides a long form certification that
is traceable to NIST.

[0110]FIG. 4b is a top plan view of the cone 40 of FIG. 4a and FIG. 4c is
a side sectional view of the cone 40 taken along lines 4c-4c of FIG. 4b.

[0111]FIG. 5 is a top plan view of the optical head 12 with its top cover
plate (60 in FIG. 1) removed in order to provide an interior view of the
optical head 12. The head 12 is attached to the saddle 24 to move
therewith. Shown adjustably mounted on the bottom base plate 61 of the
head 12 are laser steering mirrors 62, 4 lasers 64 for generating laser
beams, preferably having a wavelength of 650 nm, light plane generator or
transmitter modules, generally indicated at 66, for converting the
generated laser beams into corresponding light planes, and light plane
receiver modules, generally indicated at 68.

[0112]The calibration cone has a central hole, preferably about 1/16'',
that is utilized extensively during assembly of the cone/part holder
sub-assembly to the slide base unit as illustrated in FIG. 4c.

[0113]At that time, a thin rod ("calibration fine centering rod") is
inserted through the cone's central hole when the optical head is in the
"down" position. The rod defines the center of the calibration cone more
precisely than the constant diameter region-O which has a 1/8'' diameter.
When the head is in the "down" position the light planes pass below the
cone, nominally unblocked.

[0114]To use the rod, the part holder (44 in FIG. 3c) is removed from the
cone/part holder assembly. Then the rod can be inserted from the top,
through the plate 42, and through the cone 40, until it touches the
bottom plate 54. The rod is long enough that when it is fully inserted,
touching bottom plate 54, a length extends above plate 42, sufficient to
manipulate the rod by hand, moving it up or down.

[0115]Moving the "calibration fine centering rod" into and out of the
light planes allow one to determine if the light plane center line passes
through the rod. One observes the laser sensor outputs as the rod moves
into and out of the beam. When both left and right sensor outputs show a
slight reduction from the effect of the rod blocking the light beam, then
the light beam center line passes through the centering rod.

[0116]The calibration cone is manufactured with special instructions to
fabricate the central hole so that there is a "slip fit" of the
"calibration fine centering rod" in the calibration cone's central hole.
One rod is paired with each calibration cone, by the cone fabricator.

[0117]The use of the "calibration fine centering rod" makes it possible to
measure whether or not the cone/part holder center is aligned with the
optical head's 4-beam intersection point.

[0118]To move the cone/part holder with high precision and complete the
alignment/centering operation, four fine-pitch "pusher screws" are
utilized. Each "pusher screw" is mounted to the triangular base of the
slide/base unit (FIG. 1). There is one "pusher screw" for each of the
four directions of movement of the cone/part holder plate 50 on the
triangular base, North, East, South, and West.

[0119]After the cone/part holder assembly is centered to the optical head
beam center, the four "pusher screws" are tightened, maintaining the base
plate 50 position via compression. Then the "pusher screw" locking
sleeves are tightened. Then the base plate 50 hold down screws are
tightened. The "pusher screw" locking sleeves can also be secured with an
appropriate glue. The above-noted alignment process centers the
calibration cone precisely, with respect to the beam center of the
optical head.

[0120]FIG. 6 is a schematic perspective view of a safety enclosure,
generally indicated at 70, which encloses the Laser Lab basic measurement
hardware of FIG. 1 which sits inside the enclosure 70. A light curtain is
generated by a light curtain transmitter 72 and is received by a light
current receiver 74 to guard a physical access opening 76 to the Laser
Lab. The enclosure 70 and its hardware monitor circuitry (not shown)
guarantee that the moving optical head 12 will be stopped before a user
can physically make contact with it.

[0121]FIG. 7 is a side elevational view, partially in cross-section, of
the upper tooling unit 16. The long rod 32 of the tooling unit 16 can be
moved up/down by almost 9'' (for example) to accommodate a wide range of
UUT sizes when an operator manually grips a rod handle 78 mounted at a
proximal end 80 of the rod 32. The operator can release an upper or a
lower friction release clamp 82 and 84, respectively, of the tooling unit
16. Support brackets 86 of a support structure, generally indicated at
88, hold the upper tooling unit 16 to the base/slide unit 18. Rod guides
90 supported by a guide support 92 which, in turn, is supported by the
support brackets 86 and plates 94, hold the rod 32 precisely and also
allow it to slide smoothly up/down. Raising the top release clamp 82
allows the rod 32 to move down, while lowering the bottom release clamp
84 allows the rod 32 to move up. The spring loaded part clamp 34 has a
threaded hole 96 for threadably receiving and retaining one of a number
of possible tips such as the tip 35 that contacts the part 36 to be
inspected.

Laser Lab--Basic Measurements Overview

[0122]The Laser Lab is a system for measuring the dimensions of a variety
of manufactured parts and/or assembled parts such as parts manufactured
in the fastener industry. These parts are typically formed from
cylindrical stock by roll and impact die forming methods or by cutting
with lathes. The final part can have forms that are built up from basic
shape units, such as circular or tapered cylinder, threaded cylinder, or
additional simple shapes such as Trilobe cylinder or hex cylinder.

[0123]Substantially all of the measurements obtained with the Laser Lab
system are based on two basic components: (1) the height of a surface
from the light plane split line, and (2) the positions along the optical
head's stage axis corresponding to the various heights.

[0124]In addition, the Laser Lab system performs multiple measurements
from (4) different measurement directions. However, it is to be
understood that, depending on the part, the measurements can be taken
from as few as two measurement directions. In some cases, as many as five
measurement directions may be needed. This capability of being able to
obtain multiple measurements from multiple directions allows the Laser
Lab to explicitly utilize 3-D shape information, especially in the
measurement of threaded cylinders and non-cylindrical shapes.

Surface Height Relative to the Measurement Axis

[0125]In this section the measurements made by a single laser are
described with reference to FIG. 8. The subsystem that makes these
measurements is called a beam line. (4) beam lines are contained in one
optical head as shown in FIG. 5.

[0126]The light plane generator module 66 of FIG. 8 creates a light plane
from a single laser beam generated by the laser 64. The part creates
shadowed and unshadowed regions in the light plane. The light plane
receiver module's left and right receiver (Rcvr) components (i.e.,
optical detectors) convert the amount of light on the left and right
sides of the laser split line into separate left and right electrical
signals, respectively, which are digitized by receiver electronics when
the receiver electronics receives a trigger or sampling signal from
encoder electronics as described herein.

[0127]The PC analyzes the digitized signals from the receiver electronics
and computes left and right sensor heights. The measurement computation
utilizes a sensor height calibration (described later herein) to convert
the raw digitized sensor signals into calibrated heights, measured in mm.

[0128]The surface height measurement is further illustrated in FIG. 9. The
left and right sensor heights are the amount of light blockage in the
light plane, on both left and right sides. The light plane is nominally,
but not exactly, perpendicular to both the stage axis 28 and the part
axis.

Naming of Signals

[0129]In what follows the measurements that are based on light detections
in the left or right receivers are referred to as left or right sensor
signals or measurements, depending on the context. With (4) laser beam
lines there are (8) sensor signals in the system 10. When referring to
these signals in the entire system 10 the laser number is added to
specify the beam line. Thus names for the sensor signals range from
laser-1, left sensor through laser-4, right sensor.

[0130]As previously mentioned, FIG. 8 is a block diagram which illustrates
the basic beam line system components including the laser 64 which
generates a laser beam, the mirror 62 which reflects the laser beam and
the light plane generator module 66 which generates a light plane which
is projected at the part (UUT). The part blocks a portion of the light
plane. The components also include the light plane receiver module 68
having left and right receivers or photo detectors. However, it is to be
understood that instead of two detectors, a line scan camera, an LCD
camera or other optical detector device may be provided.

[0131]FIG. 9 which is a schematic perspective view of a cylindrical part
which is intersected by a projected laser light plane illustrates light
plane measurement geometry.

Stage Position of the Optical Head

[0132]The optical head 12 generates (4) spaced apart laser light planes
and is translated up and down by the moving stage system illustrated in
FIG. 10. The light planes are projected perpendicular to the Z-axis
defined by the movement of the stage (i.e., the stage axis 28). The
linear encoder 30 preferably produces one measurement trigger signal
pulse every 4 μm of stage movement. The signal is sent to the receiver
electronics module. At the receiver electronics module, each measurement
trigger signal pulse causes each sensor signal (8) to be digitized and
stored into the PC's memory.

[0133]This combination of the linear encoder electronics, the measurement
trigger signal, and the receiver electronics creates a sequential record
of sensor digitized raw signals when the stage moves from bottom to top.
The resulting record can be interpreted as a record of light blockage by
either a part or the calibration device at a known series of positions,
spaced 4 μm apart. In a preferred system the total linear length of
stage movement is about 235 mm.

[0134]The position measurement system described above measures position
intervals, but generally not repeatable positions. The moving stage is
stopped near the top and bottom travel limits by electronic limit
switches (not shown). Reaching the top or bottom travel limit switch by
the stage causes the motor 20 to stop. However the actual stopped
position is only approximate, since the limit switches are not precision
instruments calibrated to the encoder 30 and since the distance the stage
requires for stopping depends on the speed of travel. In practice this
results in an uncertainty in the stopping position that can be as large
as 500 μm.

[0135]To make a predictable starting position a light blockage signal is
analyzed to extract the index position of the beginning of the
calibration cone 40 in each sensor's digitized raw signal. The beginning
of the cone 40 is preferably formed by a 0.125'' diameter cylinder 96 of
the cone 40 about 0.2375'' long. The raw sensor signal is at a high level
as each light plane moves towards the beginning edge of the cone 40,
followed by a sharp step decrease in the response, and finally followed
by a constant response along the length of the cylinder 96. The analysis
software locates the midpoint of the sharp step in the response and use
that index position to set the position zero for the sensor stage axis
position. An example of the raw sensor signal is shown in FIG. 11.

[0136]It is found in practice that this technique can reduce the
positional uncertainty of fixed positions on the part or on the cone 40
to an amount (5 μm) that is much less than the uncertainty in the
stopping position (˜300 μm).

[0137]FIG. 10 is a schematic view, partially broken away, of various
position measurement system components together with the optical head 12
which is mounted on the stage to move therewith to scan the calibration
cone 40 (and later the part). As previously mentioned, the motor 20 is
coupled to the lead screw 22 which drives the moving stage and,
consequently, the optical head 12 in a direction along the stage axis 28
dependent on the direction of rotary motion of the lead screw 22. The
linear encoder senses the linear position of the moving stage along the
stage axis 28 and provides a corresponding output signal to the linear
encoder electronics. The electronics generate a trigger signal about
every 4 μm of stage movement along the stage axis. The trigger signal
is received and processed by the receiver electronics as previously
described.

[0138]FIG. 11 illustrates graphs of various raw sensor signals generated
at the beginning of the calibration cone 40 (i.e., the cylinder 96).
Shown are the signals from sensors laser-1, left, laser-1, right,
laser-4, left, and laser-4, right. The raw sensor signals are plotted
with the highest values at the bottom of the graphs with values
decreasing upwards. The left most step in the laser-1, left response
represents a jump of about 2650 digitization units in the raw sensor
response.

[0139]The plane of laser light from laser-4 is blocked by the calibration
cone 40 before the plane of laser light from laser-1 on this upward
moving scan. That is because the light plane for the laser-1 is the
lowest in the optical head 12 as shown in FIG. 10.

Multiple Beam Optical Head

[0140]As previously mentioned, the optical head 12 contains (4) beam line
subsystems. The subsystems are aligned on a common central axis. Looking
directly down on the optical head 12 the beam line light plane split
lines preferably intersect at a common point as shown in FIG. 12. The
angles of the beam lines, relative to the front of the optical head base
plate, are 22.5, 67.5, 112.5, and 157.5 degrees.

[0141]This arrangement, combined with the mechanical scanning of the light
planes, results in (8) outline images of the part, one per sensor. FIG.
13 shows the geometry of a typical situation when light from the (4) beam
lines (light planes) intersects a part with a circular cross section. For
each beam line, two shadow rays graze (are tangent to) the surface of the
part, marking the left and right limits of the part's shadow. The points
of intersection between the shadow rays and the part's surface are called
shadow points.

[0142]The calibrated distance between the shadow points and the light
plane split line is the sensor height as shown in FIG. 9. FIG. 14 shows
the sensor height plotted for both the left and right sensors of laser-1
for a full scan of a threaded part. This plot is essentially an
orthographic projection of the part, in a viewing direction aligned with
the laser-1, laser split line vector.

[0143]As previously mentioned, FIG. 12 is a top plan schematic view of the
modules 66 and 68 of the optical head 12 with its top plate 60 removed.
The laser split line for each transmitter module 66 is indicated as a
dashed ray which has an arrow head which illustrates the direction of
travel of the light beam and the plane of laser light.

[0144]FIG. 13 is a top plan view of a cylindrical part which is scanned by
4 planes of laser light including shadow rays which are tangent to the
part at shadow points.

[0145]FIG. 14 is a graph of sensor height data created from a threaded
part such as a standard thread plug go gage. Shown is laser-1 data; the
left sensor data is plotted on the top half of the image, the right
sensor data is plotted on the bottom half of the image.

Calibration Cone

[0146]The calibration cone 40 is a device which has a precisely
manufactured shape or outer surface which is scanned to obtain
calibration data which, in turn, is used to convert sensor raw digitized
signals to calibrated sensor height measurements. The cone 40 is a
rotationally symmetric with several distinct regions, each designed to
perform a different calibration function.

[0147]A symbolic sketch of the calibration cone's form is shown in FIG.
15. The calibration cone is designed with "usage regions", or specific
shapes designed to accomplish specific calibration goals. These regions
are listed in Table 1 below. Each "usage region" is designed to allow a
specific piece of calibration information to be extracted from the
scanned data set. The following sections briefly describe these pieces of
calibration information.

Stage Position Alignment of Different Sensors

[0148]Each of the lasers 64 is mounted in the optical head 12 at a
different height offset to the base plate 61 of the optical head 12 as
described with reference to FIG. 10. The height offsets are not as
precise as desired and depend on detailed optical and mechanical
adjustments of the optical head 12 and its optical modules 66 and 68. The
height difference between adjacent channels might be as large as 500
μm. The only way to make sure that the calibrated sensor stage
positions refer to the same physical objects at the same stage positions
is to measure a common physical position.

[0149]The precise location of the middle of the "begin cone edge" marks
the common zero (0) of each laser sensor's calibrated stage position.

Sensor Height Zero Position Alignment of Different Sensors

[0150]It is important to select a common zero position in a plane that is
perpendicular to the stage axis 28 and aligned parallel to the light
planes in the optical head 12. The position zero that is selected in the
calibration process is the intersection of the light planes with the
center of the small cylinder 96 at the beginning of the calibration cone
40 (i.e., FIG. 10).

[0151]A light plane split line defines a natural zero for the sensor
height measurement, but the (4) light plane split lines do not
necessarily intersect in a single point as illustrated in FIG. 12. The
optical head alignment process only ensures that the light plane split
lines intersect within a 1/16'' (1587.5 μm) cylinder.

[0152]Adding the position offset described above makes the lines defining
the position zero of each sensor's calibrated height intersect at the
center of the small cylinder 96 at the beginning of the calibration cone
40. Measuring the center, after calibration, typically gives a central
location that is less than 1 μm from (0,0).

Calibration Cone Aspect Vector, Relative to Stage Axis

[0153]The calibration cone's central axis is not necessarily exactly
aligned with the axis 28 of the stage motion, due to tolerance stackup on
a long path from cone 40, to part holder, to base plate, to slide
support, etc., and finally to the slide.

[0154]By measuring the sensor heights of the center of two calibration
cone regions, "const diam-1" and "const diam-2", the inclination of the
calibration cone's aspect vector relative to the stage axis 28 can be
determined.

[0156]The laser light planes are not exactly perpendicular to the
calibration cone aspect vector. In order to know the angle between the
light planes and the calibration cone 40, the "multi-step" region is
analyzed. Signal processing software can very accurately measure the
position of each one of the set of 5 step edges in the "multi-step"
region. The distance between the 5 step edges is precisely known. With
this information the angle of the light plane relative to the cone aspect
vector can be computed. This angle is important in determining exactly
how the light plane intersects the calibration cone 40 and thus in
extracting calibration information from the data.

[0158]The output of the laser scanned measurement is a record of the
sensor digitized raw signals for each sensor. To make sensor height
measurements in physical coordinates the raw signals need to be converted
to sensor heights.

[0159]Two regions on the calibration cone, "const slope-1" and "const
slope-2" provide this information. For example, the diameter of the
intersection between a laser light plane and "const slope-1" region
varies between 0.125'' and 0.750''. The exact diameter can be computed by
knowing the distance between the laser light plane and the beginning of
the "const slope-1" region, since the region is manufactured to high
precision.

[0160]Based on the diameter of intersection and the laser sensor outputs,
calibration tables may be constructed to convert digitized raw signals to
calibrated sensor heights.

Measurement of the Sensor "No Blockage" Signal Level

[0161]The raw signal level with no cone 40 or UUT in the light plane
sensor beam is also measured. For small parts it is often required to
extrapolate the sensor raw signal to sensor height conversion table to
smaller heights than are measured on the cone. The extrapolation is
carried out with less accuracy than more direct measurements, but the
extrapolation is very useful, especially for parts that are only slightly
smaller than the begin cone cylinder 96 ("const diam-0" region) of the
cone 40 or parts that are offset from the calibration cone central axis.

[0162]The "no blockage" signal level is also required in order to
correctly find the beginning of the cone 40.

[0163]Finally, excessive variability of the "no blockage" signal level is
a signature of variability of the light output of the laser 64 in a beam
line. This variability of the "no blockage" signal level is monitored to
generate a signal which indicates that the apparatus which generates the
beam line requires repair or is temporarily unable to carry out high
precision diameter measurements.

[0164]This measurement goal does not involve measurement of the
calibration cone 40. However, it is required to interpret calibration
cone analysis. It is made possible by the physical design of the part
holder base as illustrated in FIG. 3a, and by the alignment of the bottom
position of the optical head 12 on the base/slide to the part holder
base.

[0175]The system can be designed for smaller or larger parts wherein width
measurement limits could be changed. A small compact system for measuring
a range of smaller diameters could utilize a calibration cone 40 with
minimum and maximum diameters of 0.065'' and 0.500'' for example.

[0176]In another embodiment, changes to overall length of the cone 40 can
be made.

[0177]The calibration cone's "point design" is specified as a compromise
between ease of analysis and physical compactness. The lower the slope in
the "const slope" regions, the more precise the data that is extracted.
This is due to two reasons. First, dividing up a sloping region into
"bins" and then determining the raw data to height conversion factor
within an individual bin is more accurate when the mechanical diameter
varies least within the bin.

[0178]Second, inaccuracies in determining the light plane twist angle or
light planes that are not flat are multiplied into inaccuracies in the
raw data to height conversion factors by the mechanical slope of the
const slope regions in the calibration cone 40.

[0179]In yet another embodiment, changes to slope in "const slope" regions
can be made. As noted in the above-noted discussion, lower slopes in the
"const slope" regions translates to more accuracy in the sensor raw
data-to-height conversion factor tables.

[0180]A "point design" directed towards the goal of smaller parts and
towards a more compact system design might have a cone 40 with the same
length, but 3x smaller width dimensions. This would allow more precision
in measuring smaller parts with smaller width light planes.

[0181]In another embodiment, the cone 40 can be supported either "point
down" or "point up". The mounting direction does not matter, the
calibration measurement goals can be met with either orientation.
However, the mounting method should still allow a region of "no blockage"
sensor signal to be measured during each up/down scan.

[0182]In yet another embodiment, different number of steps in "multi-step"
region can be provided. The number of steps in the "multi-step" region
can be varied and the dimensions of the steps can be changed.

[0183]The calibration analysis that determines the light plane twist angle
uses the difference in position between steps detected in the left and
right sensors. Having more steps makes the determination more precise.

[0184]In another embodiment, changes to the number, position, or diameter
of the "constant diameter" regions can be made. The calibration cone
aspect vector is measured by analysis of the central axis of two constant
diameter regions, "const diam-1" and "const diam-2". Each region is the
same diameter. Determination of the location of their 3-D center allow
determination of the central axis of the calibration cone 40, from the
two 3-D center points. It is important that both regions be the same
diameter, to minimize the effect of diameter measurement errors on the
calibration cone axis vector.

[0185]The regions could be a different diameter, either smaller or larger.
There could also be more than two regions. Then a line could be fit
through 3 or more 3-D points to determine the calibration cone axis
vector. It is important that there be at least two regions, one region
does not typically determine the calibration cone axis vector.

Laser Lab Calibration Analysis Overview

[0186]What is now described is a process oriented overview of the Laser
Lab calibration procedure. In this procedure raw sensor data and
geometric descriptions of the calibration cone 40 are utilized to produce
calibration data. This data set can be used later to produce calibrated
sensor data from raw sensor data, as described in the "data processing"
section, below.

Calibration Goals

[0187]As previously mentioned, the following is a partial list of the
measurement process goals that are met by the design of the mechanical
calibration cone 40 and also by the calibration data analysis procedure:
[0188]G-1: stage position alignment of different sensors [0189]G-2:
sensor height zero position alignment of different sensors [0190]G-3:
calibration cone aspect vector, relative to stage axis 28 [0191]G-4:
light plane angle, relative to calibration cone 40 [0192]G-5: sensor
height calibration

Data Processing

[0193]After a laser lab scan of a UUT,(8) sensor digitized raw signals are
stored, one each for the left and right sensors, repeated for each of (4)
light planes. Each of these digitized raw sensor signals is a single
"vector" or indexed list. The sensor raw signal vector has an "index" for
every sample stored on the laser lab scan of the UUT; index 1999 refers
to the 1999-th sample taken during the scan. The value in the vector at
index 1999 is the value of the 1999-th raw signal sample.

Production of Calibration Data

[0194]After the raw sensor signal vectors are stored in memory of the
computer or PC, the vectors are analyzed to extract calibration
information. This information is extracted from the part of the digitized
raw sensor signals that contains the image of the calibration cone 40.
The extracted information results in a number of tables and parameters,
collectively called "calibration data".

Production of Calibrated Sensor Data

[0195]Once calibration data has been successfully calculated it is
utilized to produce a new set of vectors called "calibrated sensor data".
The new calibrated sensor data vectors contain two pieces of information
at each "index": the pair (calibrated stage position, calibrated height).

Calibrated Stage Position

[0196]Stage position for the N-th index in the calibrated sensor data
vector is the distance (in mm.) from the beginning of the calibration
cone 40 to the position where the raw sensor signal at the N-th index was
taken.

[0197]First the raw sensor stage indices (the index of the raw sensor
signal vector) are multiplied by the stage linear encoder spacing (4
μm in the current system), producing raw sensor stage positions. Then
the raw stage positions are referenced to the position of the beginning
of the calibration cone 40.

[0198]Finally the raw stage positions are corrected for laser tilt. The
tilt correction depends on the height of the sensor data point. If the
laser plane is slightly tilted, then any non-zero sensor height also
represents a slight change of stage position since the light plane, stage
axis coordinate system is not orthogonal. After the correction the
calibrated sensor height, calibrated sensor stage position coordinate
system is orthogonal. Having an orthogonal coordinate system makes later
measurement analysis much simpler.

Calibrated Sensor Height

[0199]Calibrated sensor height for the N-th index in the calibrated sensor
data vector is the distance (in mm) from the center of the calibration
cone's beginning cylinder 96 to the shadow ray that produced the raw
sensor signal at the N-th index.

[0200]As discussed herein above, the observations in the non-orthogonal
stage position, light plane coordinate system are corrected for the
effects of laser tilt.

Decimation to a Uniform Sampling Interval

[0201]The corrections for laser tilt result in a vector of calibrated
sensor data where the stage position distances between adjacent index
positions in the vector can vary around an average value of 4 μm.

[0202]Since uniformly sampled data is much easier to work with for
measurement analysis, the calibrated sensor data vector is decimated or
sampled to uniformly sampled calibrated sensor data vector for
measurement processing. In the current system the data is decimated to
the original linear encoder sample spacing of 4 μm.

[0204]Calibration analysis refers to the analysis of the raw sensor data
vectors containing an image of the calibration cone 40. The output of the
analysis is a set of tables and parameters called "calibration data".

Rough Edge Processing

[0205]Rough edge processing discovers the presence and rough
parameterization of the signal edges in the raw sensor data.

[0206]Rough edge processing attempts to find the "pattern" of edges that
identify the calibration cone 40 in the raw sensor data vector. This
pattern is schematically illustrated in FIG. 15.

[0207]Two types of edges are found. The first type of edge, a "step edge",
represents a vertical segment on the calibration cone 40. A step edge
detector finds one edge corresponding to the begin cone edge and (5)
edges corresponding to the locations of the vertical segments in the
calibration cone's multi-step region.

[0208]A second type of edge, a "slope edge" represents the location where
two straight segments join with each segment having a different
inclination to the vertical. A slope edge detector looks for slope edges
only in locations where there is not a step edge. All step edges are also
slope edges. The slope edge detector finds a first unique slope edge at
the location where "const diam-0" region meets "const slope-1" region,
and in (3) other places.

Rough Edge Processing--Outputs

[0209]rough locations for step edges. [0210]rough locations for slope
edges. [0211]confirmation that the calibration cone edge pattern is
present in the data.

[0212]If the rough edge processing step does not find the calibration cone
edge pattern, then the calibration analysis process is stopped.

[0217]The knowledge of the set of (8) begin cone step edges, one for each
sensor, completes calibration goal G-1: stage position alignment of
different sensors. This data is stored in the calibration data and
utilized to convert raw sensor data to calibrated sensor data.

Data Binning

[0218]At 4 μm per sampled point, there can be too much data to be
effectively analyzed for certain calibration processes. Data binning is
the process of dividing up a set of sampled points, grouping the set into
a smaller set of "bins", each bin containing a number of adjacent sampled
points.

[0219]For the tables relating the raw sensor data to calibrated sensor
heights binning is utilized. For example, the "const diam-1" region on
the calibration cone 40 is about 12 mm long, ranging from 3.810 mm to
15.558 mm along the cone axis. This would be about 3000 data points
without binning At the nominal bin size of 0.2 mm this works out to about
60 bins.

[0220]Another advantage of binning is that the data within the bin can be
averaged and checked for consistency.

[0221]Finally, the data bins are not constructed within a "guard" region
within 0.2 mm. of a detected edge. For the "const diam-1" region the
edges "slope edge-1" and "slope edge-2" mark the boundaries of the region
and the "guard" region assures us that the boundary data bin contains
only data from the uniformly sloping region.

Data Binning--Outputs

[0222]Four sets of data bins are produced, for each of (8) sensors:
[0223]two sets of data bins for the regions "const slope-1" and "const
slope-2". [0224]two sets of data bins for the regions "const diam-1" and
"const diam-2".

[0225]The "const slope-n" data is used in the construction of the sensor
height calibration table. The "const diam-n" data is used as input data
to the process that finds the position of the calibration cone's 0.750''
diameter cylinder, for the cone aspect angle estimation process.

Laser Roll

[0226]Laser roll processing finds the angle between a light plane and the
calibration cone 40 in each laser's calibrated sensor coordinate system.

[0227]For each laser the precise edge locations for the (5) edges in the
"multi step" region are obtained, one set for the left sensor and another
set for the right sensor. The difference between the left sensor and the
right sensor edge positions can be used as input to a least squares
estimate of the laser roll angle.

[0228]The detailed method of estimation is described in Appendix B.

[0229]The estimate of the laser roll angle assumes that the light plane is
flat.

[0232]The primary goal of the sensor blockage and tilt process is to
generate a calibration table that relates the sensor raw signal values to
the calibrated sensor heights.

[0233]Achieving the primary goal is made difficult because the calibration
cone 40 may be mounted at an angle that may not be parallel to the stage
axis 28. If the cone angle is not parallel to the stage axis 28 then the
interpretation of exactly where the light plane hits the calibration cone
40 depends on the angle between the calibration cone 40 and the stage
axis 28.

[0234]To solve this problem an iterative process was created.

[0235]First, the sensor calibration tables were created, assuming the cone
angle and stage axes 28 are parallel. Then using the newly created sensor
calibration table an estimate of the cone angle was made. The process is
repeated (4) times. The iterative process has been found to converge in
all cases. It is recommended that the mechanical alignment of the cone
aspect angle to the stage axis 28 be less than (1 degree).

[0241]The tables that correlates raw sensor data to calibrated sensor
heights may need to be extended. Sometimes a small part with a center
offset has a sensor height that is smaller than the minimum height in the
table. There are also gaps in the data, due to the presence of "guard"
regions, as discussed herein.

[0242]Data gaps are addressed by a linear interpolation method.

[0243]For sensor heights that are smaller than the minimum sensor height
in the sensor height calibration table the table is extrapolated to a
zero height. The last 10 points in the sensor height calibration table
are fit to a line. Then additional points are added to the sensor height
calibration table between the table's minimum height and a height of
zero.

[0244]The same process is carried out to extrapolate the sensor height
calibration table to the maximum sensor height allowed (0.750'').

Sensor Height Table Extrapolation--Outputs

[0245]The outputs of the process are additional sensor height calibration
table entries, generated from linear extrapolations to zero height and to
maximum height.

Sensor Height Table--Zero Height Position

[0246]For each sensor calibration table an offset is computed to ensure
that the sensor height is zero when the sensor views the calibration
cone's begin cone cylinder 96 (const diam-0 region).

[0257]The input data to the process is "calibrated part data". This data
set consists of (8) vectors, one for each photodetector or sensor. Each
vector consists of an indexed table of elements, each containing a (z,h)
pair. Each (z,h) pair measures the position of the UUT's shadow ray in a
coordinate system that represents each sensor's view of the UUT. z is a
measurement of calibrated sensor stage axis position, and represents the
distance along the stage axis between the current data point and the
stage position where the light plane hits the beginning of the
calibration cone. h is a measurement of calibrated sensor height and
represents the distance between the middle of the beginning cylinder of
the calibration cone and the shadow ray, perpendicular to the stage axis.

[0270]These intermediate data products are analyzed to produce final
estimates of the thread parameters. For example major diameter is
estimated as twice the radius of the 3-D crest cylinder. The 3-D crest
cylinder axis then depends on the precise crest/root locations. The
crest/root locations then depend on the search intervals based on rough
crest locations and pos/neg crossings, and on data from the original
calibrated part data.

[0271]Processing Restrictions

[0272]Inspection Region

[0273]The thread processing occurs between stage position limits called an
inspection region. In the Laser Lab template editor, the user specifies
the inspection region by manipulating the upper and lower stage position
limits, overlaid on an image of the part.

[0274]These limits utilize the calibrated sensor stage position so that
measurements by different lasers are aligned to the approximately similar
physical positions on the part.

[0275]The estimation of thread parameters is specified to be an average
estimate over all the data within the inspection region. In practice,
some of the intermediate data products are estimated outside of the
inspection region in order to allow estimation of all thread parameters
within the full region. For example, a wire position within the
inspection region may require a thread crest outside the inspection
region.

Measurement Assumption for the Inspection Region

[0276]The following requirements guide the user's placement of the
inspection region on the image of the part. At present the analysis
software does not detect a failure of any of the listed requirements
directly.

[0277]The first assumption is that the thread parameters be constant
throughout the inspection region. This enables the software to average
the estimates from different positions within the inspection region and
not be concerned with partitioning or segmenting the data into different
regions for special processing.

[0278]This requirement excludes the following types of data from the
inspection region: [0279]the beginning or end of a threaded region, with
thread crests less than full height. [0280]a threaded region with a
taper. [0281]a threaded region with a notch or extensive damage.

[0282]A second assumption is that the inspection region contains at least
4-6 thread pitches. This amount of data is required to construct several
of the intermediate data products with the required accuracy. The
intermediate data product that is most closely tied to this requirement
is the 3-D peak cylinder described herein.

[0283]A third assumption is that the thread be manufactured with a
60-degree flank angle. Thread processing implicitly utilizes this
parameter in several places. One of the most direct usages is the
conversion of lead deviation into functional diameter. Other flank angles
or other thread form shapes would require different procedures.

[0284]A fourth assumption is that the thread has a cylindrical cross
section. Non-cylindrical threads would require the 3-D peak cylinder to
be suitably generalized. Incorrect fit to a non cylindrical cross section
would lead to incorrect lead deviation measures in the current
implementation.

[0285]A fifth assumption is that the thread has a single helix. Currently
double threads are not supported.

[0286]The software does not check the assumptions. Failure to meet the
requirements will typically lead to bias in the thread measurement, or in
a failure to successfully measure the inspection region.

[0292]The thread model described hereinbelow is a sampled representation
of one sensor's thread profile, for exactly one pitch. The thread model
starts at the midpoint of a rising thread flank and ends one pitch later.

[0293]Using a correlation detector the thread model is matched to data
within the inspection regions, producing thresholded detections within
the inspection region, that are called crossings. FIG. 17 shows a sketch
of a thread model matched to the sensor data.

[0294]Later processing "refinements" noted herein may make the crossings
more accurate. The refinements also separate the crossings into positive
crossings (right flank line in FIG. 17) and negative crossings (left
flank line in FIG. 17). FIG. 18 illustrates selected concepts of a thread
form. The thread model is a lateral sequence of points that represent a
best estimate of the outline of one cycle of the thread form.

[0298]A pitch estimate is required for step set gage wire diameter. The
estimate is required to be accurate enough to unambiguously select a
unique gage wire from the set appropriate for the measurement. The
current process utilizes a two-stage process.

[0299]This process may be simplified as described herein.

[0300]First Estimate

[0301]Crossing data is analyzed and averaged over all sensors to create a
thread pitch estimate, the "crossing pitch".

[0302]Second Pitch Estimate

[0303]The steps: set wire gage diameter, wire position search intervals,
measure flank lines and measure 3-point diameters noted hereinbelow are
completed in a first iteration. Then the wire positions are averaged over
all sensors and positions to compute a pitch estimate.

[0304]Set Gage Wire Diameter

[0305]Gage wires are utilized in physical thread measurements of pitch
diameter in the prior art. Two wires are placed in adjacent threads on
one side of the UUT, and a single wire is placed on the other side of the
UUT. A micrometer measures the distance between the reference line
established by the two adjacent gage wires and the reference point
established by the other gage wire. A tabulated correction formula
converts the micrometer distance to an estimate of the pitch diameter.

[0306]Gage wire sizes are thus selected prior to the thread measurement.
To do this one estimates the thread pitch as previously described and
then one selects the closest gage wire in a set to the pitch estimate.
The gage wire set utilized is the one appropriate to the type of
measurement; currently there is one set for the metric coarse thread
sequence, and another for a similar English thread set. The gage wire
sets are chosen at part template edit time, by making a selection in a
pull down list.

[0307]Wire Position Search Intervals

[0308]One places "virtual" gage wires onto the calibrated sensor data
throughout the inspection region. In order to place the "virtual" gage
wires we must identify search intervals for each wire to be located.

[0309]A requirement of the following processing steps is that the wire
positions in the inspection region have no gaps. Another requirement is
that a wire position search interval consist of two valid thread crests,
one valid thread root between the two thread crests, and valid
positive/negative crossings between the crest/root pairs.

[0310]One then searches the set of positive/negative crossings and
crest/root positions for the set of wire position search intervals to
analyze. The result is a set of intervals, one set per sensor.

[0311]Measure Flank Lines

[0312]FIG. 19 shows a sketch of a portion of calibrated sensor data in a
single wire position search interval.

[0313]The specification of a valid wire position search interval means
that the form of the calibrated sensor data is approximately as shown in
FIG. 19. This form was used to create a plan to robustly extract flank
line data.

[0314]For the left flank line (example) we analyze all data between the
rough positions of the left crest and the central root. One then
determines the height limits of a flank line data extraction region that
covers 70% (a configurable parameter) of the height interval between left
crest and central root. This data is extracted into a data set and fit to
a line, becoming the left flank line.

[0315]The procedure avoids the non-linear regions near the left crest and
central root. In addition a "flank line valid" flag is computed, based on
the RMS distance between the left flank line and the data within the left
flank line data extraction region. If the RMS distance between the flank
line and the data points in the flank line data extraction interval is
larger than 10 μm per point (a configurable parameter), then the flag
is set to invalid.

[0316]The process is repeated for the right flank line and then for all
wire position search intervals.

[0317]Measure Wire Positions

[0318]The wire positions are calculated, given the left and right flank
lines and the wire size. As shown in FIG. 19, the virtual wire is tangent
to each flank line and the resulting position is calculated with a simple
geometric formula.

[0319]The position has a valid flag that is true when both flank lines are
valid, and false otherwise.

[0320]Measure 3-Point Diameters

[0321]The 3-point technique is a method to measure the minor, major, and
pitch diameters without explicitly utilizing 3-D information. All
computations are carried out in the 2-D laser sensor coordinate system.

[0322]For example, consider the major diameter. It is defined as the
diameter of a cylinder that contains all the inspection region's thread
crests.

[0323]In this method, the top of a thread crest in calibrated sensor
(stage position, height) coordinates forms an elementary measurement. The
elementary measurements are combined into triplets for further analysis.
Only crests from the two sensors of a single laser are combined.

[0324]Two adjacent thread crest positions in sensor-1 are combined with
the thread crest position in sensor-2 that is closest to the average
position of crests in the first sensor. The two crests in sensor-1 form a
reference line. Then the distance from the reference line to the crest in
sensor-2 is computed. This is the 3 crest distance for that crest
triplet.

[0325]In this manner, the 3-crest distances from all adjacent crest
triplets are computed, for all laser data. The 3-crest distances are all
added to a data vector. The 3-crest diameter measurement is either the
average or the median of all the 3-crest distances within the 3-crest
data vector.

[0326]3-Point Minor Diameter

[0327]The 3-point minor diameter computes 3-point distances using precise
root locations in the sensor data. The 3-point minor diameter is the
average of the 3-point distance vector.

[0328]3-Point Major Diameter

[0329]The 3-point major diameter computes 3-crest distances using precise
crest locations in the sensor data. The 3-point major diameter is the
median of the 3-point distance vector.

[0330]3-Point Wire Pitch Diameter

[0331]The 3-point pitch diameter computes 3-point distances using the wire
positions computed in the sensor data. The 3-point wire pitch diameter is
the median of the 3-point wire pitch diameter. FIG. 20 is a schematic
view which illustrates a 3-point distance method, applied to thread wire
positions. Shown are two wire positions in the top thread form with a
reference line drawn between them. Also shown is a single wire position
on the bottom thread form with the 3-point distance indicated.

[0332]FIGS. 21 and 22 are screen shots from a user interface of a PC which
illustrate intermediate data extracted from a M16×1.5 thread plug
gage. FIG. 22 is an enlarged view with its focus on a single thread
pitch.

[0333]Measure 3-D Crest Cylinder

[0334]The measured thread crest position data is analyzed to obtain a 3-D
cylinder with least squares methods. A mathematical description of the
method is given in Appendix C.

[0335]The 3-D crest cylinder fit has several output parameters of
interest: [0336]the RMS distance between the crest position data and the
fitted shape. [0337]the 3-D location of the cylinder's central axis.
[0338]the radius of the cylinder.

[0339]Project Wire Positions onto 3-D Crest Cylinder Axis

[0340]Measured wire positions can be combined with the 3-D location of the
3-D crest cylinder's central axis. An imaginary disk, perpendicular to
the cylinder axis that goes through the measured wire position marks a
position on the 3-D crest cylinder axis.

[0341]A data set consisting of the projections of all sensor wire
positions is constructed.

[0342]For a perfect helical thread and for perfectly measured wire
positions the spacing between the positions in the projected wire
positions should be exactly P/8, where P is the pitch of the thread. The
(8) sensors each give a view that is rotated 1/8 revolution between
adjacent sensors.

[0344]The output intermediate data is a vector, sorted from minimum to
maximum sensor stage position of the projected wire positions. In
addition each wire position data item is annotated with labels that
specify the laser and sensor that produced the data item and other labels
containing additional information.

Thread Parameter Estimation

[0345]Thread parameter estimation utilizes the intermediate data products
and may also correct them based on a model of the measurement, prior to
producing a final thread parameter estimate.

[0346]Wire Pitch

[0347]Thread pitch is estimated from the wire center intermediate data.
For each sensor data set the adjacent pairs of wire positions are used to
calculate an adjacent wire pitch, one per adjacent wire positions. For
all lasers, each wire pitch is added to a wire pitch vector.

[0348]The wire pitch estimate is the median of the elements in the wire
pitch vector.

[0349]Major Diameter

[0350]Thread major diameter is typically reported as the diameter of the
3-D crest cylinder.

[0351]If the 3-D crest cylinder fit was unsuccessful, the major diameter
is estimated in a different way, detailed below. The cylinder fit can
fail due to several factors listed here: [0352]part inclined at too great
an angle with respect to the stage axis. [0353]thread crest positions do
not fit a cylinder, the RMS fit-to-data distance is too large.

[0354]When the cylinder fit fails the major diameter is estimated from the
3-point major diameter data. This case is special because a previous
condition (cylinder fit) has already failed. We found in practice that
the cylinder fit most often failed when the threaded region was too short
or the inspection extended beyond the end of the threaded region.

[0355]Because of this bias we found that a simple median of the 3-point
major diameter data would typically be too low, most of the good 3-point
data was concentrated at the highest measurements. In this case the major
diameter estimate is the value such that 20% of the 3-point data is
higher and 80% of the 3-point data is lower.

[0356]Calibration Correction

[0357]Major diameter is also corrected by a final end-to-end calibration
of the total system. The reported major diameter is often too low, with
bias ranging from -20 μm to 0.

[0358]After diameter calibration we expose the system to a set of measured
thread plug gages. One then plots their major diameter bias as a function
of diameter and fit a simple segmented line to the bias results. These
bias fits then are entered into the system configuration file and are
used to correct the measured major diameter with the measured bias.

[0359]Minor Diameter

[0360]Thread minor diameter is estimated with the 3-point minor diameter
distance vector. The minor diameter value is the average of the elements
in the distance vector.

[0364]a) Compute the pitch diameter contact points with the thread flanks
by calculating the intersection of the wire shape with the left or right
flank lines.

[0365]b) Average the left and right points of intersection, and compute
the distance (radius) from the average point to the 3-D crest cylinder
fit axis. This is the pitch diameter radius for each wire position.

[0366]c) Calculate the average value of the pitch diameter radius for each
sensor.

[0367]d) Correct each sensor's average wire position radius for the part
projection angle, using the angle of the 3-D crest cylinder axis to the
stage axis, projected into each sensor's coordinate system.

[0368]e) Add left and right sensor corrected pitch diameter radius
estimates to produce an estimate of the pitch diameter for each laser.

[0369]f) Average the laser estimates to produce the system pitch diameter
estimate.

[0370]Correction for Part Projection Angle

[0371]The computation of pitch diameter is complicated by projection
effects. The laser light performs an almost perfect orthographic (shadow)
projection of the thread's shape. However the projection is not the same
thing as the thread cross section, which is specified in thread design
documents. The cross section is the thread shape if it were cut by a
plane going through the thread's central axis.

[0372]The difference is caused by the thread lead angle, which is in the
range of 1-3 degrees for many typical threads. The lead angle means that
the thread cross section is most accurately viewed in shadow when the
viewing direction coincides with the direction of the lead.

[0373]It is impossible to position the thread so that a shadow view of the
thread is simultaneously aligned with the top and bottom threads. For the
example of a thread with a 3 degree lead angle, tilting the thread to
align the top of the thread with the viewing angle will make the angle
between the lead and the viewing angle for the bottom thread about 6
degrees.

[0374]A correction factor was developed for this effect. If one knows the
tilt of the thread with respect to the viewing angle then you can correct
the observed pitch diameter radius for the expected bias caused by the
projection angle. This correction is precomputed and stored in a table.

[0375]For each sensor the tilt of the thread with respect to the viewing
angle can be obtained from the 3-D cylinder fit axis. Separate
corrections are applied to the left and right sensors.

[0376]Calibration Correction

[0377]Pitch diameter is also corrected by a final end-to-end calibration
of the total system. The reported pitch diameter is often too high, with
bias ranging from +5 μm to +35 μm.

[0378]After diameter calibration, one exposes the system to a set of
measured thread plug gages. One then plots their pitch diameter bias as a
function of diameter and fit a simple segmented line to the bias results.
These bias fits then are entered into the system calibration file and are
used to correct the measured pitch diameter with the measured bias.

[0379]Lead Deviation

[0380]The lead deviation estimate uses the wire pitch and the locations of
the wire positions as projected onto the 3-D cylinder fit axis.

[0381]For an ideal helical thread, the wire position projections should
result in a regular pattern along the 3-D cylinder fit axis. The
projection of the first laser-1, left, wire position should lie about
(1/8) pitch from the projection of the first laser-2, left, wire
position. Lead deviation is the deviation of that pattern from the ideal,
measured as a maximum distance of any projected wire position from the
ideal pattern.

[0382]The computation of the lead deviation estimate follows a
step-by-step procedure:

[0383]a) Create a wire position projection vector, containing all the
data.

[0384]b) Sort the wire position projection vector in order of position
along the 3-D cylinder fit axis.

[0385]c) Convert the wire positions of the elements of the vector into
degrees, by multiplying by the factor (360/pitch) and then reducing the
element values modulo 360.

[0386]d) Calculate an offset value so that the maximum absolute value of
the degree-valued element positions is minimal. For example with a lead
deviation of 0.010 mm for a 1 mm pitch thread, the absolute value of at
least one degree-value element position would be 3.60 degrees. (0.010
mm/1 mm equals ( 1/100) and 360/100 is 3.60.)

[0387]e) Convert the value from degrees to mm and report as the lead
deviation estimate.

[0388]Note that all lead deviation estimates are positive.

[0389]Calibration Correction

[0390]Errors in measurement mean that the physical measurement of a
perfect thread will have a positive lead deviation.

[0391]To attempt to correct for this effect, one measures the lead
deviation for a set of thread plug gages and plotted them as a function
of gage diameter. The most common form observed is a constant lead
deviation of 0.010 mm to 0.020 mm.

[0392]This value observed in calibration with thread gages is taken to be
a bias. This amount of bias is entered into the system calibration file
and used to correct the measured lead deviation for this measurement
bias.

[0393]Functional Diameter

[0394]Functional diameter is currently defined in practice by the fit of a
special fit gage over the thread. The special fit gage is essentially a
nut that is split in two by a plane cut through the central axis of the
nut. The two halves of the fit gage are held in a fixture that measures
the distance between the two halves. There is one special fit gage for
every thread type.

[0395]Functional diameter is defined as the pitch diameter when the
special fit gage is clamped tightly over a thread plug setting gage. When
one puts a different UUT into the fit gage the fit gage may expand
slightly, due to a summation of effects involving the differences between
the UUT and the thread plug setting gage used to setup the functional
diameter measurement. The functional diameter measurement is then the
thread plug setting gage's pitch diameter plus the additional separation
between the two fit gage pieces.

[0396]Functional Diameter--Laser Lab Estimator

[0397]In the Laser Lab, our functional diameter measurement method is an
approximation of the fit gage method. We do not perform a full 3-D analog
of the physical fit gage. Instead we have made an approximation that
involves the use of lead deviation and the shape of the thread form.

[0398]If we imagine the thread form as perfect and also having a 60 degree
flank angle then lead deviations should cause a the thread form fit gage
pieces to move apart. A single lead deviation either up or down the
thread form axis will cause a single split piece of the fitting gage to
move outward. The amount of outward movement for a 60 degree flank angle
will be equal to ( {square root over (3)}) (lead deviation). The movement
provides a clearance for both positive and negative movements of the
lead, relative to a perfect helical shape.

[0400]The thread model is a learned sequence of points that represent a
best estimate of the outline of one cycle of the thread form. The thread
model is calculated when the inspection region is specified, at template
edit time.

[0401]The measure template routine uses a pattern match algorithm with a
sine wave pattern to identify periodicity in the inspection region data.
This process determines an approximate thread pitch. The process also
calculates a starting point in the data vector for the first beginning of
the matched pattern, which is an approximation to the first midpoint of a
right flank line.

[0402]With the pitch and the starting point in hand, the measure template
routine can then calculate an average thread model. Starting with the
first sample point in the matched pattern, points that are 1,2,3, . . . ,
N pitches later in the inspection region are averaged to form the first
point of the thread model. The process is repeated for all the rest of
the points in the first matched pattern. The thread model is then stored
in the template for later use.

[0403]The following is a description of the structure of the trilobe or
trilobular estimation process.

Trilobe Signal Processing

[0404]Trilobe signal processing analyzes calibrated part data within the
inspection region and produces intermediate data products that are
analyzed by the trilobe parameter estimation process described
hereinbelow. Eight values are produced in trilobe signal processing, four
laser-n diameters and four laser-n centers.

Intermediate Data Product Description

[0405]Laser-n diameter Distance from left shadow ray to right shadow ray
[0406]Laser-n center Midpoint of interval spanned by left and right
shadow rays.

[0407]Trilobe Blank Signal Processing

[0408]For the trilobe threaded region, one wants to estimate the
parameters of a trilobe cylinder that touches all the thread crests
within the threaded region.

[0409]The laser-n diameter is the average of the mean left sensor height
and the mean right sensor height.

[0410]The laser-n center is the difference of the mean right sensor height
and the mean left sensor height.

[0411]Trilobe Threaded Region Signal Processing

[0412]For the trilobe threaded region, one wants to estimate the
parameters of a trilobe cylinder that touches all the thread peaks within
the threaded region.

[0417]One obtains thread crest locations from the thread region object,
keeping only thread crests that are within the inspection region, and
that also have height at least 95% of the median crest height. For a
valid inspection region there would then be 5-10 thread crest points per
sensor for typical usage of the trilobe feature.

[0418]To estimate the sensor heights one needs an estimation process that
is robust enough to tolerate several invalid thread crests. A preferred
process uses a "robust" line fit procedure to obtain a line fit through
the thread crests that will not be influenced by 1 or 2 invalid crest
data items. Once the "robust" line is found, the sensor height estimate
is the "robust" line's height at the midpoint of the inspection region.

[0419]Robust Line Fit Procedure

[0420]The robust line fit is a simple parameter sampling process. For
every pair of points in the data set to be fit, an evaluation line is
produced. A figure of merit for every evaluation line is produced and is
the RMS distance per point between the data and the evaluation line. The
RMS distances are sorted and the evaluation line with the median RMS
distance is chosen.

[0421]This procedure is computationally costly but can work correctly with
up to 49% of the data as "outliers."

[0422]Potential Issues with Trilobe Region Signal Processing

[0423]Inspection Region Taper May Bias Results

[0424]The estimation process is model-based and the model is a trilobe
"cylinder." Thus, a taper in the threaded region, such as near a thread
point, would provide data that the model fitting process would not be
capable of analyzing accurately.

[0425]Trilobe Threaded Region Crests Should Be Accurately Located

[0426]The thread region processing that locates the thread crest input
data for the threaded trilobe estimation process is very general and may
misfit crest shapes that do not match the thread region "crest model."

[0439]The trilobe D parameter can be estimated as the average of the
laser-n diameter measurements in the four lasers.

[0440]All the values should agree, within the margin of sensor errors.

[0441]If one measures a perfect trilobe shape gage, the differences
between the laser-n diameters and "D" are diagnostic of measurement
accuracy and bias. The RMS distance between the laser-n diameters and "D"
is a measure of diameter measurement uncertainty. The maximum difference
between "D" and laser-n diameter is a measure of the maximum per sensor
diameter measurement bias.

[0442]Iterative Computation of K, Angle, xCenter, yCenter Parameters

[0443]The computation of the K, Angle, xCenter, and yCenter parameters
uses only the laser-n center intermediate data product. The four laser-n
center data items are exactly enough items to compute the four unknown
trilobe parameters, there is no redundancy.

[0444]A direct four parameter search process is difficult. The search was
simplified to an iterative two parameter search with the following
analysis.

[0445]If one assumes that the (xCenter, yCenter) centerline coordinates of
the trilobe shape are known, one can estimate K, Angle with an exhaustive
search process, described hereinbelow. Once one has estimates of D, K,
and Angle, one has a complete description of the trilobe shape.

[0446]With the trilobe shape description one can calculate the different
projections of the trilobe shape onto the left and right sensors. With
the left and right sensor projections of the trilobe shape one can use
the laser-n center data to estimate the trilobe centerline coordinates,
xCenter and yCenter.

[0447]Finally, with the trilobe centerline coordinates one can change the
origin of the coordinate system specifying the laser-n center data so
that the origin of the next set of laser-n center data is at the trilobe
centerline coordinate estimate.

[0448]Then the process is repeated with the transformed laser-n center
data as input. In this process, the K, Angle search progress is presented
with data that eventually has a centerline that is very close to (0,0).
At that point, one knows all the trilobe parameters, K, Angle, D,
xCenter, and yCenter.

[0451]The K, Angle search is carried out by exhaustive enumeration. A
2-dimensional grid is constructed with 1-dimension being the possible
discrete values of K in the interval (0 . . . kmax) and the other
dimension being the possible discrete value of angle in the interval (0 .
. . 60) degrees. At each grid point K, Angle, xCenter, yCenter are used
to calculate the laser-n center values that would have produced those
values and then an RMS distance between the calculated and actual laser-n
center values.

[0454]The K, Angle search is increased in precision by a subdivided
search. A rectangular region of K, Angle space equal to a 2×2 grid
in the original K, Angle discrete grid is subdivided into a 25×25
grid and searched.

[0455]Then the process is repeated a second time, subdividing the fine
grid in the same manner

[0456]The result is a more accurate K, Angle calculation at much less cost
than a brute force search through a 3906×3906 grid. (The cost is
about 3× times a 25×25 grid search.)

[0457]Determine xCenter, yCenter

[0458]Once K and Angle are known a new estimate for the trilobe centerline
coordinates can be obtained.

[0459](1) Estimate sensor height difference caused by trilobe shape, for
all four lasers. This difference is a function of the difference between
the laser and trilobe shape angles.

ΔH(laser, trilobe)=f(K, Angle-laserAngle).

[0460](2) Correct the sensor height difference for the trilobe
contribution.

[0469]Most of the differences arise from the fact that standard thread
processing utilizes a cylinder of circular cross section whereas trilobe
thread processing utilizes a cylinder with a trilobe cross section.

[0472]The scanning optical head system described above produces a sampled
image of the amount of light and shadow in a particular sensor's beam. A
sample is produced each 4 μm of stage travel. The absolute stage
position is not precise or repeatable, as also discussed.

[0473]In order to make the stage position coordinates refer to a common
physical position, the sensor signal is analyzed to find the position of
the step edge that marks where the sensor passes the beginning of the
calibration cone 40 at the cylinder 96.

[0474]Once the sensor stage positions are all referenced to the common
begin cone position, the positions of all other features are repeatable
to high accuracy from scan to scan.

[0477]After the table relating the raw signals to sensor heights is
constructed, the table is used to compute the center of the cone 0.125''
beginning cylinder 96. That position is used as an offset to make the
calibrated sensor heights read out zero at the center of the 0.125'' cone
cylinder 96. This process establishes a common (x,y) center reference
coordinate for each of the (4) light planes.

[0478]Calibration Cone Design

[0479]Measure 3d Alignment to Stage Axis with Analysis of Two Constant
Diameter Regions

[0480]The calibration cone 40 has two regions of 0.750'' diameter that
define a cylinder in space that is concentric with the calibration cone's
central axis. By measuring the position of the 0.750'' cylinder as seen
by the sensors, the calibration software determines the alignment of the
stage axis 28 and the calibration cone axis.

[0481]It is important that the regions measured to define the calibration
cone aspect vector have the same diameter. That means that errors in the
sensor height calibration have a minimal influence on the accuracy of the
aspect vector computation.

[0484]Signal processing software measures the precise location of each of
the (5) steps. When the light plane has a roll angle with respect to the
calibration cone 40, the difference in position between the step
positions computed from a laser's left and right sensors is proportional
to the sine of the roll angle.

[0485]The analysis software utilizes the data from all (5) steps in a
least square minimization procedure that computes the roll angle.

[0487]Previous experimental designs of calibration cones used stepped
edges for the purpose of relating raw digitized sensor signals to
calibrated sensor heights. If the sensor response varied between the
height of two adjacent steps then the calibration process would not
directly measure the variation and the resulting calibration might make
mistakes at intermediate diameters.

[0488]The present design provides data at all sensor heights, in the
diameter range 0.125'' to 1.500''.

[0489]Light Plane "Layer Cake"--Reduce Cross Talk

[0490]As previously mentioned, the (4) laser light planes are arranged
parallel to the bottom plate 61 of the optical head 12, in a regularly
spaced array of heights. Adjacent laser light planes are preferably
separated by about 2.5 mm The arrangement is shown schematically in FIG.
10.

[0491]This "layer cake" arrangement was chosen specifically to eliminate
or reduce "cross talk" between different laser beam lines. For example,
light from beam line-1 might scatter from the surface of the UUT and go
into the sensor for beam line-2.

[0492]The primary means of interference is due to scattering from
cylinders that are aligned with the stage axis 28, a geometry similar to
the geometry of FIG. 9. (Also see Appendix A.)

[0493]When the laser light planes are at different heights, light from
laser-2 (for example) which is scattered by the UUT, arrives at the
sensor for laser-1 at a height of 2.5 mm relative to the expected light
from the laser-1 light plane. This scattered light can be blocked by a
light plane receiver aperture slit as described in Appendix A with
reference to FIG. 24 (i.e., telecentric apertured stops).

[0494]Light Plane Receiver Aperture Slits--Reduce Cross Talk

[0495]The light plane receivers 68 each have linear slit apertures, about
1.5 mm high, that accept light from its corresponding light plane
generator. Each aperture slit is mounted in the optical head 12 at a
different height, matched to the height of its corresponding light plane.
Light from different light plane generators or transmitters 66, scattered
by the UUT, is effectively blocked, thereby increasing measurement
accuracy.

Light Plane Receiver Aperture Pinholes

Reduce Forward Scattered Light

[0496]Each light plane receiver 68 includes photodiodes which are each
fitted with circular apertures that make the light plane receiver 68
"telecentric". This aperture pinhole accepts light rays from the nominal
angle of incidence and/or from angles of incidence that are only slightly
different (<1-2 degrees). This means that light beams that enter the
light plane receiver 68 at larger angles of incidence will be blocked by
the pinhole mask and not recorded by the measurement circuitry. Appendix
A describes this.

[0497]The pinholes reduce systematic measurement errors caused by shiny
cylindrical parts. For those parts forward scattered light will tend to
systematically reduce the diameter measured because scattered light that
would be blocked by a rough dark surface finds its way into the light
plane receiver 68.

Light Plane Generator Module

Alignment Method to Ensure Low Beam Divergence

[0498]The Laser Lab measurement system 10 has a requirement that the light
rays from each light plane generator module 66 be parallel and not
divergent.

[0499]The apparent diameter of a 0.500'' [12.7 mm] cylinder should not
change by more than 0.0001'' [0.0025 mm] as the cylinder center is moved
(+/-) 0.0394'' [1 mm] from the center of the measurement area. This
requirement couples a required measurement accuracy bias (0.0001'' [0025
mm]) with an estimated accuracy of part placement by customers
(+/-0.0394'' [1 mm]).

[0500]This requirement places limits on the alignment accuracy of the
light rays within the light plane, or its divergence. In the worst case,
the beam through the center of the cylinder is at an angle of zero, the
left shadow ray is at an angle of -φ, and the right shadow ray is at
an angle of +φ. This would mean the maximum misalignment angle for
any shadow ray in the light plane is less than 1.3 mrad.

[0501]These maximum misalignment angles translate to an accuracy of focus
when manufacturing or assembling the light plane generator module 66. An
align and focus instrument or alignment fixture, generally indicated at
100 in FIG. 31, provides a high precision mechanical adjustment of the
position of lens 316 (i.e., FIG. 37) so that the beam divergence is
minimized when constructing each light plane generator module 66. Once
the adjustments are complete, the adjustments can be permanently fixed in
place by tightening adjustment screws and by gluing the mechanical
attachment points to prevent movement. A fuller description of the
alignment method is provided in Appendix D.

Light Plane Generator

Alignment Method to Ensure High Light Beam Flatness

[0502]The Laser Lab measurement system 10 also has a requirement that the
light plane be generally flat. It was discovered that if the optical
elements of the light plane generator module 66 were misaligned then the
light plane's image on a flat target would be curved, rather than
straight.

[0503]A curved light plane would make the light plane-to-calibration cone
angle calibration described above invalid. A curved light plane would
also make the sensor height calibration described above inaccurate. The
curve in the light plane would make predicting the diameter of the
calibration cone 40 as a function of stage position much less accurate
and make the sensor height calibration much less accurate.

[0504]The align and focus method as noted above and as described in
Appendix D is designed to allow the light beam flatness of the light
plane generator module 66 to be effectively minimized during module
production. It was found that the angular and rotational alignment of the
lens 310, 312 and 316 to the module base plane was an important variable.
These alignments when performed sequentially allow the light plane
generator module 66 to be setup to meet the flatness requirement, at
which point the adjustments are permanently fixed in place by tightening
adjustment screws and gluing mechanical attachment points to prevent
movement.

[0505]Flatness is eliminated primarily by the adjustment of the lens 316,
FIG. 37, the "first cylinder lens. " The align and focus instrument
contains a rotating arm, with a clamp 188, FIG. 31.

[0506]The rotating arm's clamp holds the plate 318, FIG. 37, attached to
the first cylindrical lens 316, during alignment of the transmitter
module.

[0507]Rotation of the clamp 188 causes the laser line image at the target
210 to transition between line shapes on the target of curved upwards,
flat, and curved downwards.

[0508]A secondary adjustment is by rotating the lens 316. This adjustment
primarily affects the inclination of the laser line image at the target
210, not the curvature. The inclination is adjusted to make the laser
line image horizontal.

[0509]The adjustment of lenses 316 and 312 is inter-dependent.

Light Plane Receiver

Alignment Method

[0510]The light plane receiver modules 68 also have alignment
requirements. The optical elements of the modules 68 are precisely
positioned so that they precisely focus the light from their respective
light plane generator module 66 within the pinhole apertures of the
detectors (i.e., FIG. 24).

[0511]The receiver module 60 accepts the light from all possible light
rays within the light plane at approximately the same efficiency, so the
generator/receiver subsystem (i.e., 66 and 68, respectively) will have a
smooth light acceptance profile, as a function of distance across the
light plane. This is a requirement from the sensor height calibration
process.

[0512]The light plane receiver modules 68 and the light plane generator
modules 66 are capable of working together when mounted on the optical
head base plate 61 at standard hole positions.

[0513]The receiver light plane split line is centered within the light
plane. The align and focus method allows for the proper assembly and
subsequent testing of the light plane receiver module 68 and its
components as described herein.

[0514]Referring now to FIGS. 37-44, there is illustrated a common mirror
lens mount, generally indicated at 304, of the transmitter modules 66.
Preferably, the mount 304 is manufactured from a single piece of low
expansion al-mag alloy to ensure dimensional stability over a wide
temperature range. The mount 304 is precisely machined to ensure that the
various reference surfaces of the mount 304 are properly positioned with
respect to each other.

[0515]FIG. 37 is an exploded perspective view of the mount 304 together
with its various supported mirrors 306, a first cylindrical lens 316, a
second cylindrical lens 312 and a third cylindrical lens 310. The mount
304 includes an integrally formed lens holder 314 for the second
cylindrical lens 312. The third cylindrical lens 310 is mounted on a
front reference surface 311 of the mount 304. The second cylindrical lens
312 is held within its holder 314 which is at least partially defined by
a reference surface 313 as best shown in FIG. 61. The first cylindrical
lens 316 is held within an adjustable mounting assembly, generally
indicated at 320, which includes a base plate 319 and a lens mount 318.

[0516]A rear one of the mirrors 306 is held within an adjustable mirror
mount 322 which is mounted at a back reference surface 323 of the mount
304 (i.e., FIG. 39). The front one of the mirrors 306 is mounted
internally within the mount 304 at an internal reference surface 325 as
best shown in FIGS. 43 and 39. Comparing FIGS. 39 and 43 (sectional views
of the mount 304 without and with the mirrors 306 and the lenses 306, 310
and 312, respectively), the rear mirror 306 is mounted with respect to
the inclined reference surface 323 whereas the internally mounted mirror
306 is mounted within the module 304 with reference to the inclined
reference surface 325.

[0517]The mirrors 306 are preferably made of BK-7 material whereas the
lenses 310, 312 and 316 are made of SF-11 material. The lenses 310, 312
and 316 are optimized for a laser beam wavelength of 650 nm. Also, the
nominal affective focal lengths for the lenses 310, 312 and 316 are 107
mm, 154 mm and 2.75 mm, respectively.

[0518]The following sequence of assembly steps for the transmitter module
66 are followed, which steps are described in detail in Appendix D:

[0519]1. Secure lens 316 to lens mount 318 using a UV adhesive [0520]2.
Secure lens 312, lens 310 and front mirror 306 to lens and mirror mount
304 using UV adhesive [0521]3. Secure back mirror 306 to adjustable
mirror mount 322 using UV adhesive [0522]4. In sequence: [0523]position
rear mirror 306 mounted to adjustable mirror mount 322 within lens and
mirror mount 304 so that a reference laser beam entering the sub-assembly
parallel to the mounting base and at a height of 0.984''±0.004'' exits
the mounting aperture at the front reference surface 311 of the mount 304
at a height of 1.679''±0.020'' and remains parallel to the mounting
base within 0.05° [0524]position lens 312 so that the reference
laser beam deviates less than 0.03° [0525]position lens 316
rotationally perpendicular to the beam axis so that the beam is visually
flat [0526]position lens 316 along the beam axis so that the beam
divergence in the horizontal axis is within ±0.10 m radians
[0527]position lens 312 rotationally perpendicular to the mounting
surface of lens and mirror mount 304 so that the beam is parallel to the
assembly mounting base within 0.25° [0528]position lens 310 so
that the reference laser beam is 1.679'' from the assembly mounting
surface when measured at a distance of 10.55'' [0529]5. Completed
assembly specifications [0530]beam tilt perpendicular to laser beam
axis--less than 0.50° [0531]beam tilt parallel to laser beam
axis--less than 0.10° [0532]beam height--1.679''±0.020''
[0533]beam divergence--less than 0.25 m radians [0534]6. Secure all
fasteners and adjustable components using epoxy adhesive

[0535]Referring now to FIGS. 45 and 46a through 46d, there is illustrated
in detail the laser steering mirrors 62 and their associated component
parts illustrated in FIG. 5. The various relative positions of the laser
steering mirrors 62 as they are adjustably mounted at the top surface of
the base plate 61 are illustrated in FIGS. 46a, 46b, 46c and 46d wherein
each mirror 62 is mounted at a different angle with respect to its
respective translation plate 400. In turn, each of the translation plates
400 is adjustably mounted to the plate 61 at its top surface by mounting
screws (FIG. 5). Mounting screws 402 and their associated washers 403
secure flanges 404 of their mirror mounts, generally indicated at 406, to
their translation plates 400. In turn, the mirror mounts 406 are located
on their plates 400 at different angular positions, as illustrated in
FIGS. 46a through 46d. Each plate 400 includes an elongated aperture 408
which allows the plates 400 to be adjustably positioned at precise
angular positions on the top surface of the plate 61. Pins 410 are
provided for precise mounting the mirror mounts 406 to their plates 400
and mirror mounting plates 412 to their respective mirror mounts 406.
Screws 414 and their associated washers 416 are provided for securing the
mount plates 412 to the mirror mounts 406. The mirrors 62 are secured to
their mount plates 412 by an adhesive.

[0536]Referring now to FIG. 47, which is an exploded perspective view of
one of the light plane receiver modules 68. There is illustrated in FIG.
47 a mirror and lens mount, generally indicated at 500. The mirror and
lens mount 500 is substantially identical to the mirror and lens mount
304 of each of the light plane generator modules 66. The receiver module
68 also includes a photodetector mount, generally indicated at 502, a
lens mount receiver pair, generally indicated at 504, and a cylindrical
lens 506, which is substantially identical to the cylindrical lens 312 of
the light plane generator module 66. The receiver module 68 further
includes a pair of spherical lenses 508 which are fixably secured at a
front surface of the lens mount 504 in spaced relationship by an
adhesive. The receiver module 68 still further includes a pair of
circular apertured elements 510 and a detector PCB assembly mount 512 on
which a pair of photodetectors 514 are mounted in spaced relationship.
The lenses 508 typically are designed to operate at a wavelength of 650
nanometers, are made of SF-11 material and have a nominal affective focal
length of 25.8 mm.

[0537]The photodetector mount 502 includes upper and lower halves 501 and
503, respectively, which are secured together by screws 505.

[0538]Typically the lenses 508 are secured to the lens mount 504 with a UV
adhesive. Then the lens mount 504 is secured to the mount 500 by screws
516 and their associated washers 518, as illustrated in FIG. 5.
Typically, the lens 506 is adhesively secured to a front reference
surface of the mount 500, as also illustrated in FIG. 5, by following the
sequence of steps noted below.

[0539]The apertured elements 510 are secured within spaced holes in the
photodetector mount 502 so that the elements 510 are intimate with or
immediately adjacent to the supported detectors 514 and centered within
the mount 500. The following sequence of assembly steps are followed
which are described in detail in Appendix D: [0540]Position lens 506 at
the front reference surface of the mount 500 so a reference laser beam
entering the subassembly is parallel to a bottom reference surface of the
base of the mount 500 at a height of preferably 1.679''±0.004'' and
deviates less than 0.03°. [0541]Position lens 506 rotationally
perpendicular to its mounting surface of the mount 500 so that the
reference beam frame is parallel to the mounting base within 25°.
[0542]Position the receiver lens pair 508 within the mount 500 along the
beam axis so that the energy as measured using a reference pair of
detectors is balanced within 2%; and [0543]Position by resulting detector
assembly so that the energy measured using the mounted detectors 514 is
balanced within 1%.

[0544]After the above-noted steps are performed, all the fasteners and
adjusted components are secured using an epoxy adhesive.

[0545]The detector PCB assembly mount 512 is secured at the back surface
of the mount 500 by screws 520 and their respective spacers 522.

Glossary

[0546]3-Point distance: Distance from a single point in one laser's
sensor to a reference line produced by two points in the laser's other
sensor. [0547]3-Crest diameter: Diameter measurement produced from a
statistical average or median of 3-point distances. [0548]Align and focus
instrument: An optical/mechanical fixture for manufacturing or assembling
the light plane generator and light plane receiver modules to the
required optical tolerances. [0549]Base/slide unit: The physical base for
the Laser Lab measurement hardware. The unit includes a relatively large,
heavy triangular base and a vertical unit containing a motor, a slide,
and a linear encoder. [0550]Beam line: A set of optical, mechanical, and
electrical components contained inside an optical head that create a
light plane from a single laser beam and convert the shadowed light plane
to electrical signals for transmission to the PC tower. [0551]Calibrated
part data: A data structure consisting of calibrated sensor data from (4)
lasers, each having left and right sensors. [0552]Calibrated sensor data:
A data structure containing a vector of (stage position, height)
measurements for one sensor. [0553]Calibration cone: A precisely
manufactured single piece of tool steel with a "cone-shaped" outline. The
outline includes cylindrical and frustum-shaped outlines. The calibration
cone or device is used within the system to convert sensor digitized raw
signals to physical measurements. [0554]Calibration data: The set of
tables and parameters that are computed during the calibration analysis
process from sensor raw data. This data set is also an input to the
sensor data calibration process that converts sensor raw data to sensor
calibrated height data. [0555]Correlation detector: A signal processing
algorithm that matches a pattern with a vector data set. [0556]Diameter
calibration: The process of creating calibration data sets for each
sensor. Each data set allows analysis software to convert the sensor
digitized raw signal to a calibrated sensor height. The process is
typically a two stage process including sensor height calibration and
final diameter correction. [0557]Final diameter correction: A final
correction to diameter measurements based on a system-level diameter gage
pin calibration. [0558]Flank angle: The angle between the thread's flank
and the thread's axis. [0559]Flank line--data extraction region: The
region containing the central (70%) portion of sensor data for one flank
line. [0560]Inspection region: All the calibrated part data between two
stage position limits, start and end positions. [0561]Intermediate data:
Data produced by signal processing of calibrated sensor data that is not
saved in the template, but utilized in estimates of thread parameters.
[0562]Intermediate data--correlation crossing: A match point from the
correlation between a pattern and (typically) sensor height data.
[0563]Intermediate data--flank line: The line fit to data within the
flank line data extraction region. [0564]Intermediate data--crest/root:
Measurements of the locations (stage pos, height) of the crests and roots
present in the thread form. [0565]Intermediate data--thread 3-D cylinder:
Measurement of the 3-D cylinder formed from the least squares fit of the
thread crests. The cylinder has parameters that include its diameter, per
point RMS distance between data and fit, and cylinder axis.
[0566]Intermediate data--wire position search interval: Region that
contains two adjacent thread crests, a thread root and two thread flanks.
[0567]Laser number: The enumeration of the (4) lasers in the optical
head. The lasers are numbered laser-1 through laser-4. [0568]Light plane
generator or transmitter module: Physical module with optical and
mechanical components that converts a light beam generated by a laser
into a plane of light having parallel light rays. [0569]Light plane
receiver module: Physical module with optical, mechanical, and electrical
components that converts a plane of light having parallel light rays into
left and right electrical signals. [0570]Light plane split line:
Imaginary line that splits a light plane into left and right parts or
portions. The line is defined in the light plane receiver module and
represents the response to a shadow in the light plane in the left/right
receivers. [0571]Left/right receiver (Rcvr): The receiver is a component
of the light plane receiver module that converts light energy incident on
its surface (i.e., image plane) into an electrical signal. The signal
current is basically proportional to the amount of incident light.
[0572]Left/Right sensor digitized raw signal: The output of the light
plane receiver electronics, after processing of the left or right
receiver signals, preferably including current-to-voltage conversion,
amplification, analog filtering, and digitization. [0573]Light plane
receiver electronics: System module that converts left and right
electrical signals from the laser line receiver module into digitized
left and right raw sensor signals, and stores the results in the PC
memory. [0574]Measurement trigger signal: Signal from linear encoder
electronics transmitted to receiver electronics. One measurement trigger
signal pulse causes all sensor signals to be sampled and stored.
[0575]Optical head: A container such as a sealed metal box containing (4)
beam lines and supporting electronics. [0576]Part holder: A mechanical
subassembly mounted to the base/slide unit. The subassembly contains the
part holder base on which the parts are received and retained while being
scanned. The part holder also holds the calibration cone in a stable
position so it can also be scanned. [0577]PC tower: The PC tower is a
chassis containing a computer, a set of control electronics modules, and
a number of power supplies. [0578]Sensor digitized raw signal: The output
after processing of a left/right receiver signal by the light plane
receiver electronics. [0579]Sensor height: The distance from the laser
plane split line to the light/dark shadow edge of a part, measured in the
sensor light plane. [0580]Sensor height calibration: The process of
acquiring and analyzing a set of data that is used to convert a sensor
digitized raw signal to a sensor height. [0581]Sensor number: The
enumeration of the (8) sensors in the optical head. The values range from
laser-1 left (L1L) through laser-4 right (L4R). [0582]Sensor raw data:
The set of data generated by the Laser Lab sensor system during one scan
of the calibration cone and the UUT. [0583]Sensor calibrated height data:
The Laser Lab sensor raw data set generated from one scan, converted to
physical units and corrected for all known issues. [0584]Shadow ray: The
ray of light that just grazes the surface of the UUT. [0585]Stage axis:
The direction in 3-D space defined by the up/down movement of the optical
head's mechanical stage. [0586]Stage axis, sensor zero position: The
position of the beginning of the calibration cone, as determined by
analysis of the sensor digitized raw signal. [0587]Stage axis, calibrated
sensor position: The raw position of the stage axis, corrected to be 0 mm
at the sensor zero position. The calibrated sensor positions are
different between sensors, even between the left and right sensors for
the same laser. [0588]Stage axis, raw position: The stage axis raw
position is the value of the linear encoder counter, maintained by the
linear encoder electronics module. The zero position is set when the
stage stops during a "home" command; the linear encoder electronics
module senses a bottom trip signal, stops the motor, and then zeroes the
encoder counter. This process produces a final resting physical stage
position that varies by several hundred microns, depending on the length
of the move, the stage speed, and other factors. This physical position
is generally too uncertain for direct use in specifying positions on a
part. [0589]Thread model: An estimate for one cycle of the repeated
thread form, learned at template edit time. [0590]Thread
parameter--functional diameter: Estimate of the diameter of a virtual nut
with the nominal pitch diameter, that could contain the observed 3-D
thread form with all its lead deviations and other deviations from
perfect form. [0591]Thread parameter--lead deviation: The maximum
deviation of the lead position from the perfect helical form.
[0592]Thread parameter--major diameter: The diameter of a cylinder
enclosing all the thread crests. [0593]Thread parameter--minor diameter:
The diameter of a cylinder through all the thread roots. [0594]Thread
parameter--pitch diameter: The diameter of a cylinder that intersects the
perfect thread form midway between crest and root. [0595]Thread
parameter--pitch: The average or median axial distance between adjacent
threads. [0596]Upper tooling: A mechanical subassembly mounted to the
base/slide unit. The upper tooling includes a long stainless steel rod
that can be moved up and down to hold a variety of different-sized parts.
This tooling also includes a spring-loaded part clamp that facilitates
placement, retention and release of parts. [0597]Unit under test (UUT):
The part being measured.

[0598]While embodiments of the invention have been illustrated and
described, it is not intended that these embodiments illustrate and
describe all possible forms of the invention. Rather, the words used in
the specification are words of description rather than limitation, and it
is understood that various changes may be made without departing from the
spirit and scope of the invention.

Appendix A

Split Laser--Diameter Bias Due to Beam Scattering

Summary

[0599]scattering of light from a reflective cylinder leads to an
underestimate of its diameter. [0600]the scattering from a cone section
has a large underestimate than the cylinder. [0601]introducing a pin hole
aperture in the laser receiver can limit this effect, by making the laser
receiver telecentric, or sensitive to light from a narrow range of
incident angles.

[0602]Scattering from a Perfectly Reflective Cylinder

[0603]Scattering of light from a cylinder will cause a systematic
underestimate of the cylinder's diameter in the split laser system. Light
incident near the surface of the cylinder is scattered at a glancing
angle as illustrated in FIG. 23. If the scattered light gets into the
split laser detector, then the cylinder diameter appears systematically
smaller. The effect is largest if the cylinder is perfectly reflective,
and is absent if the cylinder is perfectly absorbing. The magnitude of
this effect is calculated to set a limit on the systematic size
underestimate.

The Scattering Angle

[0604]One can imagine a beam of light that hits the cylinder and is
reflected. The beam's direction of travel would reach a depth of ΔH
within the cylinder if the beam direction were continued on a straight
line. The beam is deflected by an angle θrefl=2θ by the
perfectly reflecting surface as shown in FIG. 23.

[0605]For a cylinder of diameter d the following relationships hold
between the scattering angle, θrefl=2θ, the depth
ΔH, and the diameter, d.

[0606]In the ideal case, with no scattering, the amount of light that
reaches the detector is from line generator light rays that do not
intersect the part. The light signal is then related to the orthographic
projection of the part, perpendicular to the beam direction.

[0607]With scattering, light that would have been blocked may enter the
detector.

[0608]The scattered light could fail to enter the detector due to one of
the following effects: [0609]light absorption at the cylinder surface
(such as by black coatings), [0610]scattering light may miss the entrance
to the laser light receivers.

[0611]One can calculate an upper limit to the underestimate of light
blockage by a perfectly reflecting cylinder. It is assumed that all light
scattered through angles smaller than angle θmax will be
received in the laser light receiver. All light scattered through larger
angles is lost and is not received in the laser light receiver.

[0614]As illustrated in FIG. 24, the simplest model of the laser receiver
includes a focusing lens for each of the left and right sensors, and a
laser diode to measure the laser light at each sensor's focusing lens'
focal plane. The laser diodes are placed at the focus for a light source
at infinite distance.

[0615]In this model, when a light ray is incident on the focusing lens at
a different angle, then the light is focused at a slightly different
place in the len' s focal plane.

[0616]The change in position at the focal plane between light incident
along the optical axis and light incident at an angle θ, is
θpos=f tan(θ)≈f(θ), where f is the focal length
of the lens.

[0617]A pinhole aperture is added in front of the laser diode to make the
laser receiver sensitive to light only in a small range of incident
angles. Similar apertures are used in the construction of telecentric
lenses. The pinhole aperture size is shown in the following table.

[0627]The Laser Lab cone fixture 40 has cone sections (i.e., frustums)
with cone angle θcone=35 degrees and
cos(θcone)=0.820. What this means in practical terms is that
the scattering angle on the cone sections is about 20% less than the
scattering angles on the cylindrical sections. The approximate scaling
developed in the previous sections indicates that the diameter
underestimate would then be about 10% more than for the straight cylinder
case.

Energy Absorption on the Cylinder

[0628]If the cylinder surface were coated with a light absorbing coating,
then it might be that the reflected light would continue on in the same
direction as the perfectly reflecting case, but with reduced intensity.

[0629]One can develop a model similar to the one noted above. In that
model all light scattered between angles 0 and θmax enters the
split laser detector of FIG. 24. Suppose that a fraction fabsorb of
the scattered light, between angles 0 and θmax, is absorbed at
the cylinder surface, and a fraction (1-fabsorb) continues on to the
a absorb, split laser detector, then the new model is:

2ΔH≈(1-fabsorb)d(θmax)2/4.

[0630]Thus, the underestimate is improved by the factor (1-fabsorb).

Appendix B

Summary

[0631]This Appendix describes in detail one embodiment of the Laser Lab
calibration process.

[0641]FIG. 26 shows a schematic outline of the sensor signal produced by
the smooth cone. The different regions of the signal are labeled and
explained below.

[0642]FIG. 26: Cone Outline--One Sensor's Signal

[0643]The signal is plotted with full open sensor level shown at the
bottom of the figure and fully blocked sensor level shown at the top of
the figure.

Full Open Region--beginning of scan to beginning of cone. [0644]establish
the full open level. [0645]establish the exact position of the beginning
of the cone.Cone Slope Data--two constant slope regions separated by a
const diameter step. [0646]establish the sensor blockage table, the
correspondence between sensor level and known diameters.Const Diameter
Data--two constant diameter data regions, 0.750'' diameter.
[0647]establish the center line of the cone. [0648]establish the position
of the cone center line, relative to the stage travel axis.Steps--Roll
Angle Data--five constant height, constant width steps. [0649]establish
the angle of the laser light plane, relative to the cone center
line.Support Bracket--no data, all light blocked.Part Support
Cylinder--constant diameter fixed support. [0650]top of this cylinder
plus height of part support cap may establish part base position.Part
Support Cap--constant diameter cap. [0651]provides base for part.
[0652]center position may vary slightly, caps are not fixed, there are
several for different types of parts, the insertion is easy for operators
to change. [0653]height and diameter are more precise than the center
position.

Cone Signal Processing

[0654]In this section the processing and analysis are described that occur
prior to creating a sensor blockage calibration table and determining the
cone tilt angle.

[0655]In this section, the raw sensor data is processed and yields
features in the (stage position, sensor level) space.

[0656]In the Laser Lab system, the conversion between stage position
encoder count and stage position is simple and requires no calibration.
The stage encoder count is multiplied by the Sampling Interval (0.004 mm)
to produce the stage position.

[0657]In this section stage positions may be specified interchangeably
using encoder counts or position values.

Data Partitioning and Consistency Checks

[0658]The first step is to partition the data into regions for more
detailed processing. With the smooth cone calibration design the
partition can be accomplished with a combination of the positions of the
step edges and known positions of features on the calibration cone.

Rough Step Edge Positions

[0659]The following step edge positions should be identified in the data.
The edges will be identified with a low precision edge finder, using
finite difference detection, with smoothing.

[0661]The following slope edge (2-nd derivative) positions should be
identified in the data. The edges will be identified with a low precision
edge finder, using finite difference detection, with smoothing.

[0666]If either of checks 1, 2, 3 fail, the calibration process is stopped
and diagnostic and logging messages are generated on the computer or PC.

Sensor Data Partition Table

[0667]The sensor data partition table gives the rough stage position of
boundaries between the calibration data regions, on a per sensor basis.
The table is used by downstream functions to provide rough starting
points for find location modules.

Full Open Signal Level

[0668]The full open signal level is computed from data in the full open
estimation region, shown in FIG. 27. The region's size is specified as
well as the size of two guard regions, for begin part and for begin scan.

[0669]The median and the order statistic corresponding to "sigma" are
computed, from the sample data within the full open estimation region.

Full Open Signal Level--Consistency Checks

[0670]c-1: The full open estimation region must fit between the two guard
regions. [0671]c-2: The "sigma" order statistic must be less than the
MaxSignalVariance parameter.

[0672]If these checks fail, the calibration fails and diagnostic and
logging messages are generated on the computer or PC.

High Precision Step Edge Positions

[0673]High precision step edge position processing uses the rough edge
step positions as initial locations to find high precision edge
parameters for 7 step edges. The step edges are the begin part edge, the
part support cylinder end step, and the 5 steps in the roll angle data.
For each step edge, 4 parameters are computed, the step position, step
height, the beam width, and a step quality measure.

[0674]The high precision edge detector uses three line fits to the step
edge data, one before the step edge, one after the step edge, and one in
the step edge transition region. A fixed size guard region,
LineFitRegionSize, keeps non-linear data out of the before/after step
line fit regions. The central LineFitCentralRegion percent of the
transition region data is used for the transition region line fit. FIG.
28 shows these data regions.

[0675]The fit degree in the before/after part regions can be adjusted to
be either const (degree-1) or linear (degree-2). For example the begin
cone step edge requires before step fit degree-1 and after step fit
degree-2. The first step edge requires before step fit degree-2 and after
step fit degree-1. Steps 2..5 require both before/after step fits to be
degree-1.

[0681][0682]c-1: all high precision step edges found, and within
MaxPosDev distance of expected positions. [0683]c-2: all step edges have
heights within MaxHeightDev of expected heights. [0684]c-3: the
RMS(data-fit) average is less than MaxRmsDev.

[0685]If these checks fail, the calibration fails, diagnostic and logging
messages are generated on the computer or PC.

Begin Cone Sensor Position Offsets

[0686]The set of high precision step edge positions at the begin cone step
edge defines a position offset for each sensor.

[0687]The position scale is defined separately for each laser sensor. The
position offset defines the 0-position for feature processing.

Laser Roll Computation

[0688]Laser roll is computed from the high precision step edge positions
found in the roll angle data region.

is related to the roll angle of the laser line, β. β is positive
when the Left sensor edge position is less than the Right sensor edge
position. β would be viewed as a counter clockwise rotation if the
left sensor height is plotted as a positive number and the right sensor
height is plotted as a negative number:

ΔstepPos(laser,i)=sin(β)Diam(i).

[0693]The 5 equations relating ΔstepPos(laser,i) to Diam(i), can be
expressed as the matrix vector equation below:

Asin(β)=b.

[0694]The data can be reduced with a least squares solution of the matrix
vector equation, producing a single parameter estimator, βest

[0696]c-1: Verify that the RMS distance per data point between the data,
{ΔstepPos(laser,i)}, and the values predicted from the fit,
{sin(βest)Diam(i)}, is less than MaxDeltaStepPosVariance.

Process Sensor Blockage Data

[0697]Raw sensor readings are processed in the cone slope region to
produce a set of (sensor level, stage position) features. Typically each
feature is based on a small region (10-500 samples) of data. The features
are only generated in regions of valid data, for example they are kept
away from step edges by a guard region.

Identify Cone Slope Data Regions

[0698]The first cone slope data region lies between the begin cone step
edge and the beginning of a const diameter data angle region. The second
region starts at the end of the same const diameter cone aspect angle
region and extends to the position of the first roll angle step.

[0699]Approximate per sensor boundaries of the cone slope data regions are
available from the Sensor Data Partition Table, computed in Data
Partitioning and Consistency Checks step.

Data Binning

[0700]Data is only binned further than GuardRegion counts from the cone
slope region boundaries, to prevent systematic diameter calibration
offsets.

[0701]The regions are divided into the number of bins specified by the
RegionBinSize and the number of sample positions in the cone slope
region. If the number of available sample positions is not divided by
RegionBinSize, then extra samples are added to the guard regions.

[0702]The data bins are not overlapping.

Data Averaging/Feature Generation

[0703]Sensor data observations within the bin are processed, forming
estimates of sensor levels and variance within the bin.

[0704]Position data within the bin are processed to form a bin position
average.

[0710][0711]c-1: Each data bin's RMS(fit-data) estimator should be less
than MaxDataBinSigma. [0712]c-2: Deviation of each data bin's fit value
from the value predicted by linear estimation using two adjacent bins
(left and right) should be less than MaxDataBinLinDev. The end bins are
tested using extrapolation from two left or two right bins.

[0717]What is actually required is the projection of the cone 40 onto the
light beam, as a function of the stage position. This projection depends
on two angles, the laser roll angle β, and the cone tilt angle a,
the angle between the stage travel axis and the cone symmetry axis.

[0718]Each of the angles is defined per laser, the full set of laser and
cone angles is {Bi, αi}, where the laser index-i is in
the interval (1..4).

[0719]The laser roll angle is known, but the cone tilt angle must be
calculated. Since the cone tilt angle is small, typically less than
1-degree, an iterative process can be successfully defined.

[0720]Initially one can assume that cone tilt angle a is equal to zero.
With this assumption one can use the cone model to generate the expected
projection of the cone onto the sensor as a function of stage position.
The set of cone projections paired with corresponding sensor responses is
used to make the Sensor Blockage Table. After construction, the table
gives the amount of material blocking the sensor, as a function of the
expected sensor response.

[0721]The sensor blockage table is then utilized to process the constant
diameter region data, producing an estimate of the cone tilt angle
α.

[0734]Signal processing of the cone slope data regions produces a list of
features, one for each sensor data bin. The feature specifies the stage
position and the average sensor level within the data bin:

[0750]Signal processing of the cone's constant diameter data regions
produces a list of features, one for each sensor data bin. The feature
specifies the stage position and the average sensor level within the data
bin:

{SensorBlockagei}={StagePositioni, SensorLeveli].

[0751]The Sensor Blockage Table is used to compute features from SB
features.

[0752]The calibrated constant data region features are utilized to compute
a linear fit to the sensor height data as a function of
StagePositionOffseti=SPi.

[0753]Left and Right sensor data are fit simultaneously for each laser,
since the cone tilt angle a affects both. Tilt angle a positive causes
the sensor heights to increase in the Left sensor and decrease in the
Right sensor.

[0754]The laser roll angle widens the step profile, and also biases the
position.

Cone 3-D Location Analysis

[0755]The 3-D cone direction unit vector, {right arrow over (α)}, is
observed in each of the 4 laser systems as the cone tilt angle. The
projection of the cone unit vector, {right arrow over (α)}, into
the laser system "I" is (αix, αiy, αiz).

[0756]In FIG. 30, α1, α2, α3,
α4 are the projections of {right arrow over (α)} on the
y'-axis for the θ=22.5, 67.5, 112.5, 157.5 degree laser sensor
systems. The positive y'-axis is the Right sensor and the negative
y'-axis is the Left sensor direction.

[0757]The x,y components of the cone unit vector, {right arrow over
(α)}, are projected into laser system "i" with the following
equation (the z component along the stage axis is unchanged):

[0762]Calibration model analysis is an iterative process. The number of
iterations computed is MaxIterations. After computing the last iteration
the Iteration Control stopping criteria is evaluated to determine if a
valid Sensor Blockage Table was constructed. (See consistency checks c-3
and c-4 below.)

Consistency Checks

[0763]c-1: Sorting {SensorBlockagei} features by increasing
StagePosition should produce a SensorLevel list sorted in decreasing
order. This ensures that one can produce a Sensor Blockage Table that
predicts a unique sensor blocking height at every valid SensorLevel.
[0764]c-2: The maximum absolute value of the interpolation error is less
than SensorLevelMaxInterpolationError. [0765]c-3: The number of
iterations is less than or equal to MaxIterations. [0766]c-4: The change
in Sensor Blockage Table projection between the last two iterations
Δδi=δi.sup.(n)-δi.sup.(n-1) is
less than MaxSensorProjectionChange. [0767]c-5: The maximum absolute
deviation of {SensorHeighti} from the value predicted in the cone
angle fit is less than MaxRmsHeightDev. [0768]c-6: The maximum absolute
deviation of the four cone tilt angles {αi} from values
predicted by the fit to the 3-D unit vector {right arrow over (α)}
is less than MaxRmsConeAngleDev.

Analysis and Error Propagation

[0769]In this section one can see how known errors in the elementary data
items, such as step edge positions and median sensor values, affect the
system measurements.

Effect of Sensor Blockage Table Errors Due to Roll Angle on Diameter Bias
Due to Position Offsets

[0770]Suppose that during calibration there was a roll angle error. Then
the left sensor actual height would be overestimated and the right sensor
actual height would be underestimated and systematic diameter measurement
errors would occur. In this situation, the placement offset of the center
of a cylindrical object from the center of the calibration axis would
cause a systematic offset in the measured diameter.

[0771]Roll angle that is too large causes a underestimate of the correct
projection of the cone for the left sensor and an overestimate for the
right sensor.

HL=HL0(1-ε)

HR=HR0(1+ε)

[0772]The measured diameter can then be shown to have a systematic offset
that is proportional to the roll angle error.

D=HL+HR=HL0(1-ε)+HR0(1+ε)

D=(HL0+HR0)-(HL0-HR0)ε

D=D0+ε(CtrPosition), where the center position is defined as

CtrPosition=(HL0-HR0).

[0773]The relative error in the diameter measurement, due to the roll
angle error is:

δ D D = δ ( tan ( β ) ) tan
( γ ) ( CtrPosition D ) . ##EQU00013##

Effect of Roll Angle Errors on Sensor Blockage Table

[0774]Roll angle errors couple with the cone slope to systematically
(example) overestimate the left sensor actual height and underestimate
the right sensor actual height, in the Sensor Blockage Table.

HL=HL0(1-tan(β)tan(γ))

[0775]For a small roll angle:

δH=±H0 tan(γ)δ(tan(β))

For H=0.500'', γ=350, δH=0.0001'', one should have
δβ≦0.3 mrad=0.016 degree.

Effect of Step Edge Location Errors on Roll Angle

[0776]A simple method to find the roll angle finds the position of two
step edges, and computes the angle from the difference in step positions.

tan(β)=(ΔStepPos)/Diameter

δ(tan(β))=δ(ΔStepPos)/Diameter

[0777]The difference in two uncorrelated step positions has approximately
40% greater uncertainty than a single step position.

δ(tan(β))=δ(StepPos) {square root over (2)}/Diameter

[0778]Step positions have σ≈0.005 mm, and at a
Diameter=1.400'', this works out to σ(tan(β))≈0.2mrad.

Effect of Multiple Scan Calibration on System Diameter Bias

[0779]The system diameter bias has an error distribution that is similar
to the repeatability distribution for a diameter measurement.

[0782]In general, this appendix describes how to fit a cylinder to a set
of points. The set of points could be determined in any manner The
application within Laser Lab is the fitting of a cylinder to the set of
"thread crest" locations. This cylinder is used to estimate the thread
region's major diameter. The data for a thread crest cylinder measurement
is a set of (stage z coordinate, sensor height) pairs. These data points
are the observed locations of the thread crests. For a 6-pitch thread
inspection region, the number of data points per thread is 4 (lasers)*2
(sensors)* 2 (flanks)* 6 (pitches)=96 (data points).

[0783]One would then like to fit all the data points to a simple linear
model of the thread crest cylinder in space with 5 free parameters:
[0784]the cylinder radius, R. [0785]the slope of the cylinder center
line, in x and y, (αx, αy). [0786]the (x,y) center
of the cylinder center line, at the beginning of the inspection interval,
(bx, by).

[0792]For example, the data set for the laser-2, right sensor is
{(zi(2, R), hi(2, R))}.

[0793]To fit one laser's data, one can develop a linear matrix equation.
The parameters are: [0794]a(l)=slope of cylinder line in laser-l
coordinates. [0795]b(l) =intercept of cylinder line in laser-l
coordinates. [0796]r=radius of cylinder.

Note: Z specifies the matrix containing data from all 4 lasers, and Z(l)
specifies the data matrix containing data from just one laser.

[0804]The new 4 laser equation can be solved by standard least squares
techniques. We will show the solution to develop the structure of the
ZT Z and ZT

[0805]H matrices. We don't actually solve for this set of 9 parameters in
practice. In the next section, we will transform the equation to
eliminate the dependencies among the a(l) and b(l) parameters, and reduce
the number of unknown parameters to 5.

[0808]The result is quite simple, a matrix containing the accumulated
sensor positions, the accumulated sensor positions squared, and the
number of measurements for laser l. For this case, you don't have to sum
separately for L and R sensors.

[0810]This is also simple, containing the difference between left and
right positions and the difference between the number of left and right
data items. Separate L and R sensor sums are required here.

[0811]Third, the vector ZT(l)h(l).

[0812]This vector contains the correlation between heights and stage
coordinates and the accumulated sum of sensor heights.

[0823]The parameters a(i), b(i) specified as 8 parameters of the above
9-parameter fit are actually projections of the 3-D thread axis
parameters ax, ay, bx, by. A projection matrix, P,
defines the mapping from the 5-parameter fit to the 9-parameter fit. The
angles ai are the angles of the laser beams with respect to the
stage (x,y) axes. For example, laser-1 is incident at 22.5 degrees,
laser-2 at 67.5 degrees, laser-3 at 112.5 degrees, and laser-4 at 157.4
degrees.

[0845]Every term is the dot product of a geometrical vector representing
the incident angles of each of the 4 lasers and a data vector
representing what is observed in each of the 4 lasers. There are 15
independent numbers to be calculated for (PT(ZT Z)P) and 5 for
PT(ZT H).

[0846]Suppose the cylinder is exactly aligned with the z-axis, and the
data is exactly centered above and below each sensor's center line. Then
in each sensor, the left sensor will have a measurement points of type
(x, h) and the right sensor will have measurements of type (x, -h). This
will mean that the sHZ and sH terms will be zero. The term ΔsH will
be a sum of +h and -(-h) terms or ΔsH≈Nh.

[0847]The method of assembly and alignment of the optical and mechanical
components of the light plane generator receiver and light plane
generator modules is executed utilizing the alignment fixture 100 of FIG.
31. The alignment fixture includes an optical rail, modified breadboards
120, 151 and 180, and a plurality of custom and commercially available
opto-mechanical positioning devices. A detector assembly generally
indicated at 159 and located on breadboard 151 replicates the function of
a light plane receiver module.

[0848]The alignment fixture 100 further includes a references laser 118,
aligned such that the center line of its light beam is horizontally
parallel to the reference breadboards 120 and 151 and lens and mirror
mount 304 interfaces. Preferably, the laser 118 is a spatially filtered
solid state laser such as the 40001 available from LumenFlow Corp. of
Middleville, Mich.

[0849]Also shown supported on the breadboard 120 is a reference prism
(i.e., rhomboid prism) assembly 122, the use of which is detailed herein.
As it is the method of assembly that is important to proper functioning
modules, this disclosure emphasizes the interfacing components of the
fixture while focusing on the detailed steps of the method to produce the
modules.

[0850]Alignment fixture 100 of FIG. 31 includes a rail and a plurality of
stage assemblies, generally indicated at 102, which, in turn, includes a
rail assembly, generally indicated at 104. The rail assembly 104 includes
a rail 106 which is supported by pairs of bench legs 108. The alignment
fixture 100 also includes a set of rail carriages 110 slidably mounted on
the rail 106.

[0860]The rhomboid prism has the property that a straight beam of light
entering the prism exists the prism in a straight line and in a path
exactly parallel to the beam path of the entry beam. The height
difference between the entry and exit beams is set by the rotational
angle of the prism, relative to the entry beam.

[0861]The alignment fixture 100 also includes a clamp post assembly 130
with a kinematic ball also supported on the breadboard 120.

[0862]The alignment fixture 100 further includes a relay telescope
assembly, generally indicated at 132, which, in turn, includes a relay
doublet assembly 134 and an IR doublet 136. The relay telescope 132 is
mounted on its carriage 110 by a scope mount 138.

[0863]Mounted on the rail 106 is another carriage 110 on which a post 140
is supported at one end of the fixture 100. The post 140 supports a
filter or target holder 142. In turn, the holder 142 supports a target
210.

[0864]The alignment fixture 100 also includes a receiver stage assembly
150 which, in turn, includes the breadboard or substrate 151. The
receiver stage assembly 150 includes an alignment assembly 152 including
an alignment aperture 154.

[0865]The receiver stage assembly 150 further includes an optical rail 156
supported on the breadboard 151. The detector assembly, generally
indicated at 159, is adjustably mounted on the rail 156. The detector
assembly 159 includes electronic boards 157, together with the sensor
mount with an aperture 155. The receiver stage assembly 150 further
includes a filter mount 160 and a vertical slit 162.

[0866]The receiver stage assembly 150 also includes a manipulator bracket
164 and an assembly 166 having a kinematic ball 168 mounted at a distal
end thereof.

[0868]The L1 manipulator stage assembly 113 includes the breadboard 180 on
which an x-y stage assembly 184, together with a kinematic base 186 are
mounted. Adjustment screws 182 are provided to adjust the position of the
stage assembly 184. Clamping arms 188 are mounted at a distal end of the
stage assembly 113.

[0869]FIGS. 31 and 37 taken together illustrate: a clamp for lens 310
attached to screw 116; a screw attached to clamp 114 below plate 120; a
rotational screw to rod attached to clamp 188; lower two screw holes of
322 establish a rotational adjustment axis for 322; and upper one screw
hole of 322 establishes control of rotation about 322 adjustment axis.

Overview of the Transmitter Alignment Process

[0870]The alignment of the transmitter module's optical components is
important to the operation of the Laser Lab system. The alignment is
accomplished with the align and focus (i.e., A&F) instrument or fixture
100.

[0871]The transmitter module is mounted in the A&F instrument 100, using
reference surfaces "A", "B", and "C" of the module. Reference surface "A"
mounts flat to base plate 120, which has been aligned parallel to the
laser beam used in the A&F instrument 100, generated by the laser 118.
Reference surface "C" is made flush to two kinematic mounts that have
been aligned parallel to the laser beam 118. Reference surface "B" is
flush to one kinematic mount and establishes the correct position along
the beam line of the laser 118.

[0872]Mirror-2 is mounted to reference surface "E" on module 304. Mirror-1
is mounted to plate 322 which is mounted to reference surface "F".

[0873]In the alignment process the optical components are configured to
meet system requirements using the process previously detailed. During
the alignment process the A&F instrument 100 holds the parts in place
with a set of clamps and piece holders. When the alignment process is
completed, the optical components are fixed to the surfaces of module 304
with a glue that permanently holds them in place.

[0874]The following is a list of transmitter optical module components and
their adjustments that are fixed in the align and focus instrument 100:
[0875]first cylindrical lens 316: plate 318 is clamped in A&F part 188.
188 can be moved in three directions: linearly parallel to laser beam and
linearly perpendicular to laser beam (with screws 182), and rotated about
the axis of the rod holding clamp 188. [0876]second cylindrical lens 312:
linearly perpendicular to laser beam with screw below plate 120, attached
to 114. The lens can also be rotated within clamp 114. [0877]third
cylindrical lens 310: linearly perpendicular to laser beam, in up-down
relation to plate 120, by screw 116. The lens can also be rotated within
clamp attached to screw 116. [0878]adjustable mirror mount 322: radial
adjustment about axis established by lower two screws mounting 322 to
304. Adjustment controlled by top screw of 322.

[0879]Thus, there are a total of eight independent adjustment parameters
for the transmitter module. Each parameter is optimized in the alignment
process, and fixed with glue before removal of the module 66 from the A&F
instrument 100.

1.0 Produce light plane generator module assembly: [0880]1.1 Install
machined lens and mirror mount 304 to breadboard 120. Place an alignment
aperture in the reference laser beam path so that the laser beam passes
through the aperture without creating distortion. [0881]1.2 Install lens
312 within the mount 304 and align so that the beam passes through the
aperture and is coincident with the laser beam axis. Remove the aperture
from the beam path. [0882]1.3 Assemble L1 sub-assembly 320 components,
cylinder lens 316, L1 lens mount plate 318 and lens mount base 319.
Assemble sub-assembly 320 to mount 304. [0883]1.4 Adjust rotational
position of lens 312 about the laser beam axis so that beam profile is
level (FIG. 32). [0884]1.5 Adjust rotational position of lens 316 about
the one degree of freedom provided so that the beam profile is flat (FIG.
33). [0885]1.6 Adjust position of lens 316 along laser beam axis so that
beam width is visually equal at a distance greater than or equal to 216''
and at 4'' distance from exit aperture of machined lens and mirror mount
304. [0886]1.7 Adjust the horizontal position of lens 316 so that power
distribution is visually balanced within the beam profile. [0887]1.8
Re-adjust the horizontal position of lens 312 so that beam is centered
along the laser beam axis. [0888]1.9 Re-adjust position of lens 316 along
laser beam axis so that full angle laser beam divergence is less than
0.25 m radians.

[0888]θ=(d2-d1)/(l2-l1) [0889]where θ is beam divergence in
radians; d1 is the horizontal beam width at distance 1; d2 is the
horizontal beam width at distance 2; l1 is the distance from the output
aperture of machined lens and mirror mount 304 to the beam measurement
point at distance 1; l2 is the distance from the output aperture of
machined lens and mirror mount 304 to the beam measurement point at
distance 2. [0890]1.10 Assemble lens 310 to machined lens and mirror
mount 304.